Ccent Icnd1 100-101 Glossary Terms

Front 1

10BASE-T

Back 1

The 10-Mbps baseband Ethernet specification using two pairs of twisted-pair cabling (Categories 3,4, or 5): One pair transmits data and the other receives data. 10BASE-T, which is part of the IEEE 802.3 specification, has a distance limit of approximately 100 m (328 feet) per segment.

Front 2

100BASE-T

Back 2

A name for the IEEE Fast Ethernet standard that uses two-pair copper cabling, a speed of 100 Mbps, and a maximum cable length of 100 meters.

Front 3

1000BASE-T

Back 3

A name for the IEEE Gigabit Ethernet standard that uses four-pair copper cabling, a speed of 1000 Mbps (1 Gbps), and a maximum cable length of 100 meters.

Front 4

802.1Q

Back 4

The IEEE standardized protocol for VLAN trunking.

Front 5

802.1a

Back 5

The IEEE standard for wireless LANs using the U-NII spectrum, OFDM encoding, at speeds of up to 54 Mbps.

Front 6

802.1b

Back 6

The IEEE standard for wireless LANs using the ISM spectrum, DSSS encoding, and speeds of up to 11 Mbps.

Front 7

802.1g

Back 7

The IEEE standard for wireless LANs using the ISM spectrum, OFDM or DSSS encoding, and speeds of up to 54 Mbps.

Front 8

802.1n

Back 8

The IEEE standard for wireless LANs using the ISM spectrum, OFDM encoding, and multiple antennas for singlestream speeds up to 150 Mbps.

Front 9

AAA

Back 9

Authentication, authorization, and accounting, Authentication confirms the identity of the user or device. Authorization determines what the user or device is allowed to do. Accounting records information about access attempts, including inappropriate requests.

Front 10

access interface

Back 10

A LAN network design term that refers to a switch interface connected to end-user devices, configured so that it does not use VLAN trunking.

Front 11

access link

Back 11

In Frame Relay, the physical serial link that connects a Frame Relay DTE device, usually a router, to a Frame Relay switch. The access link uses the same physical layer standards as do point-to-point leased lines.

Front 12

accounting

Back 12

In security, the recording of access attempts. See AAA.

Front 13

address block

Back 13

In both IPv4 and IPv6, a set of consecutive addresses. This term is typically used for public addresses, assigned by some numbering authority (IANA/ICANN, an RIR, or an ISP).

Front 14

adjacent-layer interaction

Back 14

The general topic of how on one computer, two adjacent layers in a networking architectural model work together, with the lower layer providing services to the higher layer.

Front 15

administrative distance

Back 15

In Cisco routers, a means for one router to choose between multiple routes to reach the same subnet when those routes were learned by different routing protocols. The lower the administrative distance, the better the source of the routing information.

Front 16

ADSL

Back 16

Asymmetric digital subscriber line. One of many DSL technologies, ADSL is designed to deliver more bandwidth downstream (from the central offices to the customer site) than upstream.

Front 17

All-nodes multicast address

Back 17

A specific IPv6 multicast address, FF02::1, with link local scope, used to send packets to all devices on the link that support IPv6.

Front 18

All-routers multicast address

Back 18

A specific IPv6 multicast address, FF02::2, with link local scope, used to send packets to all devices that act as IPv6 routers on the local link.

Front 19

Anti-X

Back 19

The term used by Cisco to refer to a variety of security tools that help prevent various attacks, including antivirus, antiphishing, and antispam.

Front 20

Area Border Router (ABR)

Back 20

A router using OSPF in which the router has interfaces in multiple OSPF areas.

Front 21

ARP

Back 21

Address Resolution Protocol. An Internet protocol used to map an IP address to a MAC address. Defined in RFC 826.

Front 22

ARP table

Back 22

A list of IP addresses of neighbors on the same VLAN, along with their MAC addresss, as kept in memory by hosts and routers.

Front 23

ARPANET

Back 23

The first packet-switched network, first created around 1970, which served as the predecessor to the Internet.

Front 24

asymmetric

Back 24

A feature of many Internet access technologies, including DSL, cable, and modems, in which the downstream transmission rate is higher than the upstream transmission rate.

Front 25

asynchronous

Back 25

The lack of an imposed time ordering on a bit stream. Practically, both sides agree to the same speed, but there is no check of adjustment of the rates if they are slightly different. However, because only 1 byte per transfer is sent, slight differences in clock speed are not an issue.

Front 26

ATM

Back 26

Asynchronous Transfer Mode. The international standard for cell relay in which multiple service types (such as voice, video, and data) are conveyed in fixed-length (53 byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays.

Front 27

authentication

Back 27

In security, the verification of the identity of a person or a process. See AAA.

Front 28

authorization

Back 28

In security, the determination of the rights allowed for a particular user or device. See AAA.

Front 29

autonegotiation

Back 29

An IEEE standard mechanism (802.3u) with which two nodes can exchange messages for the purpose of choosing to use the same Ethernet standards on both ends of the link, ensuring that the link functions and functions well.

Front 30

autonomous system

Back 30

An internetwork in the administrative control of one organization, company, or governmental agency, inside which that organization typically runs an interior gateway protocol (IGP).

Front 31

auxiliary port

Back 31

A physical connector on a router that is designed to be used to allow a remote terminal, or PC with a terminal emulator, to access a router using an analog modem.

Front 32

back-to-back link

Back 32

A serial link between two routers, created without CSU/DSUs, by connecting a DTE cable to one router and a CDE cable to the other. Typically used in labs to build serial links without the expense of an actual leased line from the telco.

Front 33

balanced hybrid

Back 33

A term that, over the years, has been used to refer to the logic behind the EIGRP routing protocol. More commonly today, this logic is referred to as advanced distance vector logic.

Front 34

bandwidth

Back 34

A reference to the speed of a networking link. Its origins come from earlier communications technology in which the range, or width, of the frequency band dictated how fast communications could occur.

Front 35

basic service set (BSS)

Back 35

In wireless LANs, a WLAN with a single access point.

Front 36

bitwise Boolean AND

Back 36

A Boolean AND between two numbers of the same length in which the first bit in each number is ANDed, and then the second bit in each number, and then the third, and so on.

Front 37

Boolean AND

Back 37

A math operation performed on a pair of one-digit binary numbers. The result is another one-digit binary number. 1 AND 1 yields 1; all other combinations yield a 0.

Front 38

boot field

Back 38

The low-order 4 bits of the configuration register in a Cisco router. The value in the boot field in part tells the router where to look for a Cisco IOS image to load.

Front 39

broadcast address

Back 39

Generally, any address that represents all devices, and can be used to send one message to all devices. In Ethernet, the MAC address of a binary 1s, or FFFF.FFFF.FFFF in hex. For IPv4, see subnet broadcast address.

Front 40

broadcast domain

Back 40

A set of all devices that receive broadcast frames originating from any device withing the set. Devices in the same VLAN are in the same broadcast domain.

Front 41

broadcast frame

Back 41

An Ethernet frame send to destination address FFFF.FFFF.FFFF, meaning that the frame should be delivered to all hosts on the LAN.

Front 42

broadcast subnet

Back 42

When subnetting a Class A, B, or C network, the one subnet in each classful network for which all subnet bits have a value of binary 1. The subnet broadcast address in this subnet has the same numeric value as the classful network's network-wide broadcast address.

Front 43

bus

Back 43

A common physical signal path composed of wires or other media across which signals can be sent from one part of a computer to another.

Front 44

cable internet

Back 44

An Internet access technology that uses a cable TV (CATV) cable, normally used for video, to send and receive data.

Front 45

CDP

Back 45

Cisco Discovery Protocol. A media- and protocol-independent device discovery protocol that runs on most Cisco-manufactured equipment, including routers, access servers, and switches. Using CDP, a device can advertise its existence to other devices and receive information about other devices on the same LAN or on the remote side of a WAN.

Front 46

CDP neighbor

Back 46

A device on the other end of some communications cable that is advertising CDP updates.

Front 47

CIDR

Back 47

Classless interdomain routing. An RFC-standard tool for global IP address range assignment. CIDR reduces the size of Internet routers' IP routing tables, helping deal with the rapid growth of the Internet. The term classless refers to the face that the summarized groups of networks represent a group of addresses taht do not conform to IPv4 classful (Class A, B, and C) grouping rules.

Front 48

circuit switching

Back 48

A generic reference to network services, typically WAN services, in which the provider sets up a (Layer 1) circuit between two devices, and the provider makes no attempt to interpret the meaning of the bits. See also packet switching.

Front 49

Cisco Express Forwarding (CEF)

Back 49

A method of internal processing on Cisco routers, meant to make the routing process very efficient, doing so by caching IP routes in a table that can be searched very quickly, and by remembering data link headers rather than building them for every packet that is forwarded.

Front 50

classful IP network

Back 50

An IPv4 Class A, B, or C network; called a classful network because these networks are defined by the class rules for IPv4 addressing.

Front 51

classful routing protocol

Back 51

Does not transmit the mask information along with the subnet number, and therefor must consider Class A, B, and C network boundaries and perform autosummarization at those boundaries. Does not support VLSM.

Front 52

classless routing protocol

Back 52

An inherent characteristic of a routing protocol, specifically that the routing protocol does send subnet masks in its routing updates, thereby removing any need to make assumptions about the addresses in a particular subnet or network, making it able to support VLSM and manual route summarization.

Front 53

CLI

Back 53

Command-line interface. An interface that enables the user to interact with the operating system by entering commands and optional arguments.

Front 54

clock rate

Back 54

The speed at which a serial link encodes bits on the transmission medium.

Front 55

clock source

Back 55

The device to which the other devices on the link adjust their speed when using synchronous links.

Front 56

clocking

Back 56

The process of supplying a signal over a calbe, either on a separate pin on a serial cable or as part of the signal transitions in the transmitted signal, so that the receiving device can keep synchronization with the sending device.

Front 57

codec

Back 57

Coder-decoder. An integrated circuit device that transforms analog voice signals into a digital bit stream and then transforms digital signals back into analog voice signals.

Front 58

collision domain

Back 58

A set of network interface cards (NIC) for which a frame sent by on NIC could result in a collision with a frame sent by any other NIC in the same collision domain.

Front 59

command-line interface

Back 59

Also known as command-line user interface, console user interface, and character user interface (CUI), is a means of interacting with a computer program where the user (or client) issues commands to the program in the form of successive lines of text (command lines).

Front 60

configuration mode

Back 60

A part of the Cisco IOS Software CLI in which the user can type configuration commands that are then added to the device's currently used configuration file (running config).

Front 61

configuration register

Back 61

In Cisco routers, a 16-bit, user-configurable value that determines how the router functions during initialization. In software, the bit position is set by specifying a hexadecimal value using configuration commands.

Front 62

connected

Back 62

The single-item status code listed by a switch show interfaces status command, with this status referring to a working interface.

Front 63

connected route

Back 63

On a router, an IP route added to the routing table when the router interface is both up and has an IP address configured. The route is for the subnet that can be calculated based on the configured IP address and mask.

Front 64

connection establishment

Back 64

The process by which a connection-oriented protocol creates a connection. With TCP, a connection is established by a three-way transmission of TCP segments.

Front 65

console port

Back 65

A physical socket on a router or switch to which a cable can be connected between a computer and the router/switch, for the purpose of allowing the computer to use a terminal emulator and use the CLI to configure, verify, and troubleshoot the router/switch.

Front 66

convergence

Back 66

The time required for routing protocols to react to changes in the network, removing bad routes and adding new, better routes so that the current best routes are in all the routers' routing tables.

Front 67

CPE

Back 67

Customer premises equipment. Any equipment related to communications that is located at the customer site, as opposed to inside the telephone company's network.

Front 68

crossover cable

Back 68

An Ethernet cable that swaps the pair used for transmission on one device to a pair used for receiving on the device on the opposite end of the cable. In 10BASE-T and 100BASE-TX networks, this cable swaps the pair at pins 1,2 to pins 3,6 on the other end of the cable, and the pair at pins 2,6 to pins 1,2 as well.

Front 69

CSMA/CD

Back 69

Carrier sense multiple access with collision detection. A media-access mechanism in which devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmissions from those devices for some random length of time.

Front 70

CSU/DSU

Back 70

Channel services unit/data service unit. A device that understands the Layer 1 details of serial links installed by a telco and how to use a serial cable to communicate with networking equipment such as routers.

Front 71

cut-through switching

Back 71

One of three options for internal processing on some models of Cisco LAN switches in which the frame is forwarded as soon as enough of the Ethernet header has been received for the switch to make a forwarding decision, including forwarding the first bits of the frame before the whole frame is received.

Front 72

DCE

Back 72

Data communications equipment. From a physical layer perspective, the device providing the clocking on a WAN link, typically a CSU/DSU, is the DCE. From a packet switching perspective, the service provider's switch, to which a router might connect, is considered the DCE.

Front 73

deencapsulation

Back 73

On a computer that receives data over a network, the process in which the device interprets the lower-layer headers and, which finished with each header, removes the header, revealing the next-higher layer PDU.

Front 74

default gateway/default router

Back 74

On an IP hose, the IP address of some router to which the hose sends packets when the packet's destination address is on a subnet other than the local subnet.

Front 75

default mask

Back 75

The mask used in a Class A, B, or C network that does not create any subnets; specifically, mask 255.0.0.0 for Class A networks, 255.255.0.0 for Class B networks, and 255.255.255.0 for Class C networks.

Front 76

default route

Back 76

On a router, the route that is considered to match all packets that are not otherwise matched by some more specific route.

Front 77

demarc

Back 77

The legal term for the demaraction or separation point between the telco's equipment and the customer's equipment.

Front 78

denial of service (DoS)

Back 78

A type of attack whose goal is to cause problems by preventing legitimate users from being able to access services, thereby preventing the normal operation of computers and networks.

Front 79

DHCP

Back 79

Dynamic Host Configuration Protocol. A protocol used by hosts to dynamically discover and lease an IP address, and learn the correct subnet mask, default gateway, and DNS server IP addresses.

Front 80

DHCP Client

Back 80

Any device that uses DHCP protocols to ask to lease an IP address from a DHCP server, or to learn any IP settings from that server.

Front 81

DHCP Relay

Back 81

The name of the router IOS feature that forwards DHCP messages from client to servers by changing the destination IP address from 255.255.255.255 to the IP address of the DHCP server.

Front 82

DHCP Server

Back 82

Software that waits from DHCP clients to request to lease IP addresses, with the server assigning a lease of an IP address as well as listing other important IP settings for the client.

Front 83

directed broadcast address

Back 83

An Internet Protocol address that specifies "all hosts" on a specified network.

Front 84

distance vector

Back 84

The logic behind the behavior of some interior routing protocols, such as RIP. Distance vector routing algorithms call for each router to send its entire routing table in each update, by only to its neighbors. Distance vector routing algorithms can be prone to routing loops but are computationally simpler than link-state routing algorithms.

Front 85

DNS

Back 85

Domain Name System. An applications layer protocol used throughout the Internet for translating hose names into their associated IP addresses.

Front 86

dotted-decimate notation (DDN)

Back 86

The format used for IP version 4 addresses, in which four decimal values are used, separated by periods (dots).

Front 87

DS0

Back 87

Digital signal level 0. A 64-kbps line, or channel of a faster line inside a telco, whose origins are to support a single voice call using the original voice (PCM) codecs.

Front 88

DS1

Back 88

Digital signal level 1. A 1.544-Mbps line from the telco, with the 24 DS0 channels of 64 kbps each, plus an 8-kbps management and framing channel. Also called a T1.

Front 89

DS3

Back 89

Digital signal level 3. A 44.736-Mbps line from the telco, with 28 DS1 channels plus overhead. Also called a T3.

Front 90

DSL

Back 90

Digital subscriber line. Public network technology that delivers high bandwidth over conventional telco local-loop copper wiring at limited distances. Typically used as an Internet access technology, connecting a user to an ISP.

Front 91

DSL modem

Back 91

A device that connects to a telephone line, using DSL standards, to transmit and receive data to/from a telco using DSL.

Front 92

DTE

Back 92

Data terminal equipment. From a Layer 1 perspective, the DTE synchronizes its clock based on the clock sent by the DCE. From a packet-switching perspective, the DTE is the device outside the service provider's network, typically a router.

Front 93

dual stack

Back 93

A mode of operation in which a host or router runs both IPv4 and IPv6.

Front 94

duplex mismatch

Back 94

On opposite ends of any Ethernet link, the condition in which one of the two devices uses full-duplex logic and the other uses half-duplex logic, resulting in unnecessary frame discards and retransmissions on the link.

Front 95

Duplicate Address Detection (DAD)

Back 95

A term used in IPv6 to refer to how hosts first check whether another hose is using a unicast address before the first host uses that address.

Front 96

E1

Back 96

Similar to a T1, but used in Europe. It uses a rate of 2.048 Mbps and 32 64-kbps channels, with one channel reserved for framing and other overhead.

Front 97

EIGRP

Back 97

Enhanced Interior Gateway Routing Protocol. An advanced version of IGRP developed by Cisco. Provides superior convergence properties and operating efficiency and combines the advantages of link-state protocols with those of distance vector protocols.

Front 98

EIGRP version 6

Back 98

The version of the EIGRP routing protocol that supports the IPv6, and not the IPv4.

Front 99

enable mode

Back 99

A part of the Cisco IOS CLI in which the user can use the most powerful and potentially disruptive commands on a router or switch, including the ability to then reach configuration mode and reconfigure the router.

Front 100

encapsulation

Back 100

The placement of data from a higher-layer protocol behind the header (and in some cases, between a header and trailer) of the next-lower layer protocol. For example, an IP packet could be encapsulated in an Ethernet header and trailer before being send over an Ethernet.

Front 101

encryption

Back 101

Applying a specific algorithm to data to alter the appearance of the data, making it incomprehensible to those who are not authorized to see the information.

Front 102

error detection

Back 102

The process of discovering whether a data link level frame was changed during transmission. This process typically uses a Frame Check Sequence (FCS) field in the data link trailer.

Front 103

error disabled

Back 103

An interface state on LAN switches that can be the result of one of many security violations.

Front 104

error recovery

Back 104

The process of noticing when some transmitted data was not successfully received and resending the data until it is successfully received.

Front 105

Ethernet

Back 105

A series of LAN standards defined by the IEEE, originally invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation.

Front 106

Ethernet address

Back 106

A 48-bit (6-byte) binary number, usually written as a 12-digit hexadecimal number, used to identify Ethernet nodes in an Ethernet network. Ethernet frame headers list a destination and source address field, used by the Ethernet devices to deliver Ethernet frames to the correct destination.

Front 107

Ethernet frame

Back 107

A term referring to an Ethernet data link header and trailer, plus the data encapsulated between the header and trailer.

Front 108

Ethernet link

Back 108

A generic term for any physical link between two Ethernet nodes, no matter what type of cabling is used.

Front 109

Ethernet port

Back 109

A generic term for the opening on the side of any Ethernet node, typically in an Ethernet NIC or LAN switch, into which an Ethernet cable can be connected.

Front 110

EtherType

Back 110

Jargon that shortens the term "Ethernet Type," which refers to the Type field in the Ethernet header. The Type field identifies the type of packet encapsulated inside an Ethernet frame.

Front 111

EUI-64

Back 111

Literally, a standard for an extended unique identifier that is 64 bits long. Specifically for IPv6, a set of rules for forming the 64-bit identifier, used as the interface ID in IPv6 addresses, by starting with a 48-bit MAC address, inserting FFFE (hex) in the middle, and inverting the seventh bit.

Front 112

extended access list

Back 112

A list of IOS access list global configuration commands that can match multiple parts of an IP packet, including the source and destination IP address and TCP/UDP ports, for the purpose of deciding which packets to discard and which to allow through the router.

Front 113

exterior gateway protocol (EGP)

Back 113

A routing protocol that was designed to exchange routing information between different autonomous systems.

Front 114

Fast Ethernet

Back 114

The common name for all the IEEE standards that send data at 100 megabits per second.

Front 115

filter

Back 115

Generally, a process or a device that screens network traffic for certain characteristics, such as source address, destination address, or protocol, and determines whether to forward or discard that traffic based on the established criteria.

Front 116

firewall

Back 116

A device that forwards packets between the less secure and more secure parts of the network, applying rules that determine which packets are allowed to pass, and which are not.

Front 117

flash

Back 117

A type of read/write permanent memory that retains its contents even with no power applied to the memory, and uses no moving parts, making the memory less likely to fail over time.

Front 118

flooding

Back 118

The result of the LAN switch forwarding process for broadcasts and unknown unicast frames. Switches forward these frames out all interfaces, except the interface in which the frame arrived. Switches also flood multicasts by default, although this behavior can be changed.

Front 119

flow control

Back 119

The process of regulating the amount of data send by a sending computer toward a receiving computer. Several flow control mechanisms exist, including TCP flow control, which uses windowing.

Front 120

forward

Back 120

To send a frame received in one interface out another interface, toward its ultimate destination.

Front 121

forward acknowledgment

Back 121

A process used by protocols that do error recovery, in which the number that acknowledges data lists the next data that should be sent, not the last data that was successfully received.

Front 122

four-wire circuit

Back 122

A line from the telco with four wires, composed of two twisted-pair wires. Each pair is used to send in one direction, so four-wire circuit allows full-duplex communication.

Front 123

fragment-free switching

Back 123

One of three internal processing options on some Cisco LAN switches in which the first bits of the frame can be forwarded before the entire frame is received, but not until the first 64 bytes of the frame are received, in which case, in a well-designed LAN, collision fragments should not occur as a result of this forwarding logic.

Front 124

frame

Back 124

A term referring to a data link header and trailer, plus the data encapsulated between the header and trailer.

Front 125

Frame Check Sequence

Back 125

A filed in many data link trailers used as part of the error detection process.

Front 126

Frame Relay

Back 126

An international standard data link protocol that defines the capabilities to create a frame-switched (packet-switched) service, allowing DTE devices (typically routers) to send data to many other devices using a single physical connection to the Frame Relay service.

Front 127

Frequency Hopping Spread Spectrum

Back 127

A method of encoding data on a wireless LAN in which consecutive transmissions occur on different nearby frequency bands as compared with the prior transmission. Not used in modern WLAN standards.

Front 128

full-duplex

Back 128

Generically, any communication in which two communicating devices can concurrently send and receive data. In Ethernet LANs, the allowance for botch devices to send and receive at the same time, allowed when both devices disable their CSMA/CD logic.

Front 129

full mesh

Back 129

A network topology in which more than two devices can physically communicate and, by choice, all pairs of devices are allowed to communicate directly.

Front 130

Gigabit Ethernet

Back 130

The common name for all the IEEE standards that send data at 1 gigabit per second.

Front 131

global routing prefix

Back 131

An IPv6 prefix that defines an IPv6 address block made up of global unicast addresses, assigned to one organization, so that the organization has a block of globally unique IPv6 addresses to use in its network.

Front 132

global unicast address

Back 132

A type of unicast IPv6 address that has been allocated from a range of public globally unique IP addresses, as registered through IANA/ICANN, its member agencies, and other registries or ISPs.

Front 133

half-duplex

Back 133

Generically, any communication in which only one device at a time can send data. In Ethernet LANs, the normal result of the CSMA/CD algorithm that enforces the rule that only one device should send at any point in time.

Front 134

HDLC

Back 134

High-Level Data Link Control. A bit oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO).

Front 135

head end

Back 135

The upstream, transmit end of a cable TV (CATV) installation.

Front 136

header

Back 136

In computer networking, a set of bytes placed in front of some other data, encapsulating that data, as defined by a particular protocol.

Front 137

host

Back 137

Any device that uses an IP address.

Front 138

host address

Back 138

The IP address assigned to a network card on a computer.

Front 139

host name

Back 139

The alphameric name of an IP host.

Front 140

host part

Back 140

A term used to describe a part of an IPv4 address that is used to uniquely identify a host inside a subnet. The host part is identified by the bits of value 0 in the subnet mask.

Front 141

host route

Back 141

A route with a /32 mask, which by virture of this mask represents a route to a single host IP address.

Front 142

HTML

Back 142

Hypertext Markup Language. A simple document-formatting language that uses tags to indicate how a given part of a document should be interpreted by a viewing application, such as a web browser.

Front 143

HTTP

Back 143

Hypertext Transfer Protocol. The protocol used by web browsers and web servers to transfer files, such as text and graphic files.

Front 144

hub

Back 144

A LAN device that provides a centralized connection point for LAN cabling, repeating any received electrical signal out all other ports, thereby creating a logical bus. Hubs do not interpret the electrical signals as a frame of bits, so hubs are considered to be Layer 1 devices.

Front 145

IANA

Back 145

The Internet Assigned Numbers Authority (IANA). An organization that owns the rights to assign many operating numbers and facts about how the global Internet works, including public IPv4 and IPv6 addresses. See also ICANN.

Front 146

ICANN

Back 146

The Internet Corporation for Assigned Names and Numbers. An organization appointed by IANA to oversee the distributed process of assigning public IPv4 and IPv6 addresses across the globe.

Front 147

ICMP

Back 147

Internet Control Message Protocol. A TCP/IP network layer protocol that reports errors and provides other information relevant to IP packet processing.

Front 148

IEEE

Back 148

Institute of Electrical and Electronics Engineers. A professional organization that develops communications and network standards, among other activities.

Front 149

IEEE 802.2

Back 149

An IEEE LAN protocol that specifies an implementation of the LLC sub layer of the data link layer.

Front 150

IEEE 802.3

Back 150

A set of IEEE LAN protocols that specifies the many variations of what is known today as an Ethernet LAN.

Front 151

IETF

Back 151

The Internet Engineering Task Force. The IETF serves as the primary organization that works directly to create new TCP/IP standards.

Front 152

inactivity timer

Back 152

For switch MAC address tables, a timer associated with each entry that counts time upward from 0 and is reset to 0 each time a switch receives a frame with the same MAC address. The entries with the largest timers can be removed to make space for additional MAC address table entries.

Front 153

inside global

Back 153

For packets sent to and from a host that resides inside the trusted part of a network that uses NAT, a term referring to the IP address used in the headers of those packets when those packets traverse the global (public) Internet.

Front 154

inside local

Back 154

For packets sent to and from a host that resides inside the trusted part of a network that uses NAT, a term referring to the IP address used in the heads of those packets when those packets traverse the enterprise (private) part of the network.

Front 155

interior gateway protocol (IGP)

Back 155

A type of protocol used for exchanging routing information between gateways (commonly routers) within an Autonomous System (for example, a system of corporate local area networks). This routing information can then be used to route network-level protocols like IP.

Front 156

interior routing protocol

Back 156

A routing protocol designed for use within a single organization.

Front 157

intrusion detection system (IDS)

Back 157

A security function that examines more complex traffic patterns against a list of both known attack signatures and general characteristics of how attacks can be carried out, rating each perceived thread and reporting the threats.

Front 158

intrusion prevention system (IPS)

Back 158

A security function that examines more complex traffic patterns against a list of both known attack signatures and general characteristics of how attacks can be carried out, rating each perceived thread, and reacting to prevent the more significant threats.

Front 159

IOS

Back 159

Cisco Interwork Operating System Software that provides the majority of a router's or switch's features, with the hardware providing the remaining features.

Front 160

IOS Image

Back 160

A file that contains the IOS.

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IP

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Internet Protocol. The network layer protocol in the TCP/IP stack, providing routing and logical addressing standards and services.

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IP address (IP version 4)

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In IP version 4 (IPv4), a 32-bit address is assigned to hosts using TCP/IP. Each address consists of a network number, an optional subnetwork number, and a host number. The network and subnetwork numbers together are used for routing, and the host number is used to address an individual host within the network or subnetwork.

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IP address (IP version 6)

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IP address (IP version 6) In IP version 6 (IPv6), a 128-bit address is assigned to hosts using TCP/IP. Addresses use different formats, commonly using a routing prefix, subnet, and interface ID, corresponding to the IPv4 network, subnet, and host parts of an address.

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IP network

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A network that uses the IP protocol, which is part of the TCP/IP protocol suite. IP has become the global standard for networking, which includes the entire Internet, private LANs and many of the data and voice networks of the carriers.

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IP packet

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An IP header, followed by the data encapsulated after the IP header, but specifically not including any headers and trailers for layers below the network layer.

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IP subnet

Back 166

Subdivisions of a Class A, B, or C network, as configured by a network administrator. Subnets allow a single Class A, B, or C network to be used instead of multiple networks, and still allow for a large number of groups of IP addresses, as is required for efficient IP routing.

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IP version 4

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Literally, the version of the Internet Protocol defined in an old RFC 791, standardized in 1980, and used as the basis of TCP/IP networks and the Internet for over 30 years.

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IP version 6

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A newer version of the Internet Protocol defined in RFC 2460, as well as many other RFCs, whose creation was motivated by the need to avoid the IPv4 address exhaustion problem.

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IPv4 address exhaustion

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The process by which the public IPv4 addresses, available to create the internet, were consumed through the 1980s until today, with the expectation that eventually the world would run out of available IPv4 addresses

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IPv6 neighbor table

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The IPv6 equivalent of the ARP table. A table that lists IPv6 addresses of other hosts on the same link, along with their matching MAC addresses, as typically learning using Neighbor Discovery Protocol (NDP).

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ISDN

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Integrated Services Digital Network. A service offered by telephone companies that permits telephone networks to carry data, voice, and other traffic. Often used as an Internet access technology, as well as dial backup when routers lose their normal WAN communications links.

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ISL

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Inter-Switch Link. A Cisco-proprietary protocol that maintains VLAN information as traffic flows between switches and routers.

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ISO

Back 173

International Organization for Standardization. An international organization that is responsible for a wide range of standards relevant to networking. The ISO developed the OSI reference model, a popular networking reference model.

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keepalive

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A proprietary feature of Cisco routers in which the router sends messages on a periodic basis as a means of letting the neighboring router know that the first router is still alive and well.

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L4PDU

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Layer 4 protocol data unit. The data compiled by a Layer 4 protocol, including Layer 4 headers and encapsulated high-layer data, but not including lower-layer headers and trailers.

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Layer 3 Protocol

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A protocol that has characteristics like OSI Layer 3, which defines logical addressing and routing, IPv4 and IPv6 are Layer 3 protocols.

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learning

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The process used by switches for discovering MAC addresses, and their relative location, by looking at the source MAC address of all frames received by a bridge or switch.

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leased line

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A serial communications circuit between two points, provided by some service provider, typically a telephone company (telco). Because the telco does not sell a physical cable between the two endpoints, instead charging a monthly fee for the ability to send bits between the two sites, the service is considered to be a leased service.

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link-local address

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A type of unicast IPv6 address that represents an interface on a single data link. Packets sent to a link-local address cross only that particular link and are never forwarded to other subnets by a router. Used for communications that do not need to have the local link.

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link-local scope

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With IPv6 multicasts, a term that refers to the parts (scope) of the network to which a multicast packet can flow, with link-local referring to the face that the packet stays on the subnet in which it originated.

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link-state

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A classification of the underlying algorithm used in some routing protocols. Link-state protocols build a detailed database that lists links (subnets) and their state (up, down), from which the best routes can then be calculated.

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link-state advertisement (LSA)

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In OSPF, the name of the data structure that resides inside the LSDB and describes in detail the various components in a network, including routers and links (subnets).

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link-state database (LSDB)

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In OSPF, the data structure in RAM of a router that holds the various LSAs, with the collective LSAs representing the entire topology of the network.

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LLC

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Logical Link Control. The higher of the two data link layer sublayers defined by the IEEE. Synonymous with the IEEE 802.2.

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local loop

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A line from the premises of a telephone subscriber to the telephone company CO.

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local subnet broadcast address

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IPv4 address 255.255.255.255. A packet send to this address is sent as a data link broadcast, but only flows to hosts in the subnet into which it was originally sent. Routers do not forward these packets.

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local username

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A username (with matching password), configured on a router or switch. It is considered local because it exists on the router or switch, and not on a remote server.

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logical address

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A generic reference to addresses as defined in Layer 3 protocols that do not have to be concerned with the physical details of the underlying physical media. Used mainly to contrast these addresses with data link addresses, which are generically considered to be a physical address because they differ based on the type of physical medium.

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MAC

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Media Access Control. The lower of the two sublayers of the data link layer defined by the IEEE. Synonymous with IEEE 802.3 for Ethernet LANs.

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MAC address

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A standardized data link layer address that is required for every device that connects to a LAN. Ethernet MAC addresses are 6 bytes long and are controlled by the IEEE. Also known as a hardware address, a MAC layer address, and a physical address.

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metric

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A unit of measure used by routing protocol algorithms to determine the best route for traffic to use a reach a particular destination.

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microsegmentation

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The process in LAN design by which every switch port connects to a single device, with no hubs connected to the switch ports, creating a separate collision domain per interface. The term's origin relates to the face that one definition for the word "segment" is "Collision domain," with a switch separating each switch port into a separate collision domain or segmetnt

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modem

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Modulator-demodulator. A device that converts between digital and analog signals so that a computer can send data to another company using analog telephone lines. At the source, a modem converts digital signals to a form suitable for transmission over analog communication facilities. At the destination, the analog signals are returned to their digital form.

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multilayer switch

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A LAN switch that can also perform Layer 3 routing functions. The name comes from the fact that this device makes forwarding decisions based on logic and multiple OSI Layers (Layers 2 and 3).

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multimode

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A type of fiber-optic cabling with a larger core than single-mode cabling, allowing light to enter at multiple angles. Such cabling has lower bandwidth than single mode fiber but requires a typically cheaper light source, such as an LED rather than a laser.

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name server

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A server connected to a network that resolves network names into network addresses.

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named access list

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An ACL that identifies the various statements in the ACL based on a name, rather than a number.

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NAT

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Network Address Translation. A mechanism for reducing the need for globally unique IP addresses. NAT allows an organization with addresses that are not globally unique to connect to the Internet, by translating those addresses into public addresses in the globally routable address space.

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neighbor

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In routing protocols, another router with which a router decides to exchange routing information.

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Neighbor Advertisement (NA)

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A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used to declare to other neighbors a host's MAC address. Sometimes sent in response to a previously received NDP Neighbor Solicitation (NS) message.

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Neighbor Solicitation (NS)

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A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used to ask a neighbor to reply with a Neighbor Advertisement, which lists the neighbor's MAC address.

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network

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A collection of computers, printers, routers, switches, and other devices that can communicate with each other over some transmissions medium.

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network address

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For a class B network, the network address is the first two bytes of the IP address. For a class C network, the network address is the first three bytes of the IP address. In each case, the remainder is the host address. In the Internet, assigned network addresses are globally unique.

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network broadcast address

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In IPv4, a special address in each classful network that can be used to broadcast a packet to all hosts in the same classful network. Numerically, the address has the same value as the network number in the network part of the address and all 255s in the host octets - for example, 10.255.255.255 is the network broadcast address for classful network 10.0.0.0.

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network interface card (NIC)

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A computer card, sometimes an expansion card and sometimes integrated into the motherboard of the computer, that provides the electronics and other functions to connect to a computer network. Today, most NICs are specifically Ethernet NICs, and most have an RJ-45 port, the most common type of Ethernet port.

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network number

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A number that uses dotted-decimal notation like IP addresses, but the number itself represents all hosts in a single Class !, B, or C IP network.

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network part

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The portion of an IPv4 address that is either 1, 2, or 3 octets/bytes long, based on whether the address is in a Class A, B, or C network.

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Network Time Protocol (NTP)

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A protocol used to synchronized time-of-day clocks so that multiple devices use the same time of day, which allows log messages to be more easily matched based on their timestamps.

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networking model

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A generic term referring to any set of protocols and standards collected into a comprehensive grouping that, when followed by the devices in a network, allows all the devices to communicate. Examples include TCP/IP and OSI.

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next-hop router

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In an IP route in a routing table, part of a routing table entry that refers to the next IP router (by IP address) that should receive packets that match the route.

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NIC

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Network Interface Card.

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NVRAM

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Nonvolatile RAM. A type of random-access memory (RAM) that retains its contents when a unit is powered off.

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ordered data transfer

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A networking function, included in TCP, in which the protocol defines how the sending host should number the data transmitted, defines how the receiving device should attempt to reorder the data if it arrives out of order, and specifies to discard the data if it cannot be delivered in order.

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OSI

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Open System Interconnection reference model. A network architectural model developed by the ISO. The model consists of seven layers, each of which specifies particular network functions, such as addressing, flow control, error control, encapsulation, and reliable message transfer.

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OSPF

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Open Shortest Path First. A popular link-state IGP that uses a link-state database and the Shortest Path First (SPF) algorithm to calculate the best routes to reach each known subnet.

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OSPF version 2

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The version of the OSPF routing protocol that supports IPv4, and not IPv6, and has been commonly used for over 20 years.

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OSPF version 3

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The version of the OSPF routing protocol that supports IPv6, and not IPv4.

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outgoing interface

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In an IP route in a routing table, part of a routing table entry that refers to the local interface out which the local router should forward packets that match the route.

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overlapping subnets

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An (incorrect) IP subnet design condition in which one subnet's range of addresses includes addresses in the range of another subnet.

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packet

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A logical grouping of bytes that includes the network layer header and encapsulated data, but specifically does not include any headers and trailers below the network layer.

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packet switching

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A generic reference to network services, typically WAN services, in which the service examines the contents of the transmitted data to make some type of forwarding decision. This term is mainly used to contrast with the WAN term circuit switching, in which the provider sets up a (Layer 1) circuit between two devices and the provider makes no attempt to interpret the meaning of the bits.

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partial mesh

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A network topology in which more than two devices could physically communicate by, by choice, only a subset of the pairs of devices connected to the network is allowed to communicate directly.

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patch cable

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An Ethernet cable, usually short, that connects from a device's Ethernet port to a wall plate or switch. With wiring inside a building, electricians prewire from the wiring closet to each cubicle or other location, with a patch cable connecting the short distance from the wall plate to the user device.

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PDU

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Protocol data unit. An OSI term to refer generically to a grouping of information by a particular layer of the OSI model. More specifically, an LxPDU would imply the data and headers as defined by Layer x.

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ping

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An Internet Control Message Protocol (ICMP) echo message and its reply; ping often is used in IP networks to test the reachability of a network device.

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pinout

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The documentation and implementation of which wires inside a cable connect to each pin position in any connector.

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port

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In TCP and UDP, a number that is used to uniquely identify the application process that either sent (source port) or should receive (destination port) data. In LAN switching, another term for switch interface.

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Port Address Translation (PAT)

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A NAT feature in which one inside global IP address supports over 65,000 concurrent TCP and UDP connections.

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port number

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A field in a TCP or UDP header that identifies the application that either sent (source port) or should receive (destination port) the data inside the data segment.

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port security

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A cisco switch feature in which the switch watches Ethernet frames that come in an interface (a port), tracks the source MAC addresses of all such frames, and takes a security action if the number of different such MAC addresses is exceeded.

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PPP

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Point-to-Point Protocol. A protocol that provides router-to-router and host-to-network connections over synchronous point-to-point and asynchronous point-to-point circuits.

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prefix

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In IPv6, this term refers to the number than identifies a group of IPv6 addresses. An IPv6 subnet identifier.

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prefix length

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In IPv6, the number of bits in an IPv6 prefix.

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prefix notation (IP version 4)

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A shorter way to write a subnet mask in which the number of binary 1s in the mask is simply written in decimal. For example, /24 denotes the subnet mask with 24 binary 1 bits in the subnet mask. The number of bits of value binary 1 in the mask is considered to be the prefix length.

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private addresses

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IP addresses in several Class A, B, and C networks that are set aside for use inside private organizations. These addresses, as defined in RFC 1918, are not routable through the internet.

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problem isolation

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The part of the troubleshooting process in which the engineer attempts to rule out possible causes of the problem until the root cause of the problem can be identified.

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protocol data unit (PDU)

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A generic term referring to the header defined by some layer of a networking model, and the data encapsulated by the header (and possibly trailer) of that layer, but specifically not including any lower-layer headers and trailers.

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Protocol Type field

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A field in a LAN header that identifies the type of header that follows the LAN header. Includes the DIX Ethernet Type field, the IEEE 802.2 DSAP field, and the SNAP protocol Type field.

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PSTN

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Public switched telephone network. A general term referring to the variety of telephone networks and services in place worldwide. Sometimes called POTS, or plain old telephone service.

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PTT

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Post, telephone, and telegraph. A government agency that provides telephone services. PTTs exist in some areas outside of North America and provide both local and long-distance telephone services.

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public IP address

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An IP address that is part of a registered network number, as assigned by an Internet Assigned Numbers Authority (IANA) member agency, so that only the organization to which the address is registered is allowed to use the address. Routers in the Internet should have routes allowing them to forward packets to all the publicly registered IP addresses.

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quartet

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A term used in this book, but not in other references, to refer to a set of four hex digits in an IPv6 address.

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RAM

Back 243

Random-access memory. A type of volatile memory that can be read and written by a microprocessor.

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RFC

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Request For Comments. A document used as the primary means for communicating information about the TCP/IP protocols. Some RFCs are designated by the Internet Architecture Board (IAB) as Internet standards, and others are informational. RFCs are available online from numerous sources, including www.rfc-editor.org.

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RIP

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Routing Information Protocol. An interior gateway protocol (IGP) that uses distance vector logic and router hop count as the metric. RIP version 2 (RIP-2) replaced the older RIP version 1 (RIP-1), with RIP-2 providing more features, including support for VLSM.

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RJ-45

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A popular type of cabling connector used for Ethernet cabling. It is similar to the RJ-11 connector used for telephone wiring in the homes in the United Sates. RJ-45 allows the connection of eight wires.

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ROM

Back 247

Read-only memory. A type of nonvolatile memory that can be read by not written to by the microprocessor.

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root cause

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A troubleshooting term that refers to the reason why a problem exists, specifically a reason for which, if changed, the problem would either be solved or changed to a different problem.

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routed protocol

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A protocol that defines packets that can be routed by a router. Examples of routed protocols include IPv4 and IPv6.

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Router Advertisement (RA)

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A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used by routers to announce their willingness to act as an IPv6 router on a link. These can be sent in response to a previously received NDP Router Solicitation (RS) message.

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router ID (RID)

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In OSPF, a 32-bit number, written in dotted-decimal notation, that uniquely identifies each router.

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Router Solicitation (RS)

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A message defined by the IPv6 Neighbor Discovery Protocol (NDP), used to ask any routers on the link to reply, identifying the router, plus other configuration settings (prefixes and prefix lengths).

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routing protocol

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A set of messages and processes with which routers can exchange information about routes to reach subnets in a particular network. Examples of routing protocols include the Enhanced Interior Gateway Routing Protocol (EIGRP), the Open Shortest Path First (OSPF) protocol, and the Routing Information Protocol (RIP).

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routing table

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A list of routes in a router, with each route listing the destination subnet and mask, the router interface out which to forward packets destined to that subnet, and as needed, the next-hop router's IP address.

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routing update

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A generic reference to any routing protocol's messages in which it sends routing information to a neighbor.

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running-config file

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In Cisco IOS switches and routers, the name of the file that resides in RAM memory, holding the device's currently used configuration.

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same-layer interaction

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The communication between two networking devices for the purposes of the functions defined at a particular layer of a networking model, with that communication happening by using a header defined by that layer of the model. The two devices set values in the header, send the header and encapsulated data, with the receiving device(s) interpreting the header to decide what action to take.

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Secure Shell (SSH)

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A TCP/IP application layer protocol that supports terminal emulation between a client and server, using dynamic key exchange and encryption to keep the communications private.

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segment

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In TCP, a term used to describe a TCP header and its encapsulated data (also called an L4PDU). Also in TCP, the process of accepting a large chunk of data from the application layer and breaking it into smaller pieces that fit into TCP segments. In Ethernet, a segment is either a single Ethernet cable or a single collision domain (no matter how many cables are used).

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segmentation

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The process of breaking a large piece of data from an application into pieces appropriate in size to be sent through the network.

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serial cable

Back 261

A type of cable with many different styles of connectors used to connect a router to an external CSU/DSU on a leased line installation.

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serial interface

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A type of interface on a router, used to connect to some types of WAN links, particularly leased lines and Frame Relay access links.

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setup mode

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An option on Cisco IOS switches and routers that prompts the user for basic configuration information, resulting in new running-config and startup-config files.

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shared Ethernet

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An Ethernet that uses a hub, or even the original coaxial cabling, that results in the devices having to take turns sending data, sharing the available bandwidth.

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Shortest Path First (SPF) algorithm

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The name of the algorithm used by link-state routing protocols to analyze the LSDB and find the least-cost routes from that router to each subnet.

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single-mode

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A type of fiber-optic cabling with a narrow core that allows light to enter only at a single angle. Such cabling has a higher bandwidth than multimode fiber but requires a light source with a narrow spectral width (such as a laser).

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solicited-node multicast address

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A type of IPv6 multicast address, with link-local scope, used to send packets to all hosts in the subnet that share the same value in the last six hex digits of their unicast IPv6 addresses. Begins with FF02::1:FF00:0/104.

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SONET

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Synchronous Optical Network. A standard format for transporting a wide range of digital telecommunications services over optical fiber.

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Spanning Tree Protocol (STP)

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A protocol that uses the Spanning Tree algorithm, allowing a switch to dynamically work around loops in a network topology by creating a spanning tree. Switches exchange bridge protocol data unit (BPDU) messages with other switches to detect loops and then remove the loops by blocking selected switch interfaces.

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standard access list

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A list of IOS global configuration commands that can match only a packet's source IP address, for the purpose of deciding which packets to discard and which to allow through the router.

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star

Back 271

A network topology in which endpoints on a network are connected to a common central device by point-to-point links.

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startup-config file

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In Cisco IOS switches and routers, the name of the file that resides in NVRAM memory, holding the device's configuration that will be loaded into RAM at the running-config file when the device is reloaded or powered on.

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stateful DHCP

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A term used in IPv6 to contrast with stateless DHCP. Stateful DHCP keeps track of which clients have been assigned which IPv6 addresses (state information).

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Stateless Address Autoconfiguration (SLAAC)

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A feature of IPv6 in which a host or router can be assigned an IPv6 unicast address without the need for a stateful DHCP server.

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stateless DHCP

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A term used in IPv6 to contrast with stateful DHCP. Stateless DHCP servers don't lease IPv6 addresses to clients. Instead, they supply other useful information, such as DNS server IP addresses, but with no need to track information about the clients (state information).

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static route

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An IP route on a router created by the user configuring the details of the route on the local router.

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store-and-forward switching

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One of the internal processing options on some Cisco LAN switches in which the Ethernet frame must be completely received before the switch can begin forwarding the first bit of the frame.

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STP

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Shielded twisted-pair. This type of cabling has a layer of shielded insulation to reduce electromagnetic interference (EMI).

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straight-through cable

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In Ethernet, a cable that connects the wire on pin 1 on one end of the cable to pin 1 on the other end of the cable, pin 2 on one end to pin 2 on the other end, and so on.

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subnet

Back 280

Subdivisions of a Class A, B, or C network, as configured by a network administrator. Subnets allow a single Class A, B, or C network to be used instead of multiple networks, and still allow for a large number of groups of IP addresses, as is required for efficient IP routing.

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subnet broadcast address

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A special address in each subnet, specifically the largest numeric address in the subnet, designed so that packets sent to this address should be delivered to all hosts in that subnet.

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subnet ID (IPv6)

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The number that represents the IPv6 subnet. Also known as the IPv6 prefix, or more formally as the subnet router anycast address.

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subnet mask

Back 283

A 32-bit number that numerically describes the format of an IP address, by representing the combined network and subnet bits in the address with mask bit values of 1, and representing the host bits in the address with mask bit values of 0.

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subnet number

Back 284

In IPv4, a dotted-decimal number that represents all addresses in a single subnet. Numerically, the smallest value in the range of numbers in a subnet, reserved so that it cannot be used as a unicast IP address by a host.

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subnet part

Back 285

In a subnetted IPv4 address, interpreted with classful addressing rules, one of three parts of the structure of an IP address, with the subnet part uniquely identifying different subnets of a classful IP network.

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subnetting

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The process of subdividing a Class A, B, or C network into smaller groups called subnets.

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switch

Back 287

A network device that filters, forwards, and floods Ethernet frames based on the destination address of each frame.

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switched Ethernet

Back 288

An Ethernet that uses a switch, and particularly not a hub, so that the devices connected to one switch port do not have to contend to use the bandwidth available on another port. This term contrasts with shared Ethernet, in which the devices must share bandwidth, whereas switched Ethernet provides much more capacity, as the devices do not have to share the available bandwidth.

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symmetric

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A feature of many Internet access technologies in which the downstream transmission rate is the same as the upstream transmission rate.

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synchronous

Back 290

The imposition of time ordering on a bit stream. Practically, a device will try to use the same speed as another device on the other end of a serial link. However, by examining transitions between voltage states on the link, the device can notice slight variations in the speed on each end and can adjust its speed accordingly.

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T1

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A line from the telco that allows transmission of data at 1.544 Mbps, with the ability to treat the line as 24 different 64-kbps Ds0 channels (plus 8 kbps of overhead).

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TCP

Back 292

Transmission Control Protocol. A connection-oriented transport layer TCP/IP protocol that provides reliable data transmission.

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TCP/IP

Back 293

Transmission Control Protocol/Internet Protocol. A common name for the suite of protocols developed by the U.S. Department of Defense in the 1970s to support the construction of worldwide internetworks. TCP and IP are the two best-known protocols in the suite.

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telco

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A common abbreviation for telephone company.