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250 Cards in this Set

  • Front
  • Back
10BASE-T
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
100BASE-TX
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
1000BASE-T
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.
802.1Q
The IEEE standardized protocol for VLAN trunking.
802.11a
The IEEE standard for wireless LANs using the U-NII spectrum, OFDM
encoding, at speeds of up to 54 Mbps.
802.11b
The IEEE standard for wireless LANs using the ISM spectrum, DSSS
encoding, and speeds of up to 11 Mbps.
802.11g
The IEEE standard for wireless LANs using the ISM spectrum, OFDM or
DSSS encoding, and speeds of up to 54 Mbps.
802.11i
The IEEE standard for wireless LAN security, including authentication and
encryption.
AAA
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.
access interface
A LAN network design term that refers to a switch interface
connected to end-user devices.
access link
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
access point
A wireless LAN device that provides a means for wireless clients to
send data to each other and to the rest of a wired network, with the AP connecting to
both the wireless LAN and the wired Ethernet LAN.
accounting
In security, the recording of access attempts. See AAA.
ad hoc mode
wireless LANs, a method or mode of operation in which clients
send data directly to each other without the use of a wireless access point (AP).
adjacent-layer interaction
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.
administrative distance
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.
ADSL
Asymmetric digital subscriber line. One of many DSL technologies, ADSL is
designed to deliver more bandwidth downstream (from the central office to the
customer site) than upstream.
Anti-X
The term used by Cisco to refer to a variety of security tools that help prevent
various attacks, including antivirus, anti-phishing, and anti-spam
ARP
Address Resolution Protocol. An Internet protocol used to map an IP address
to a MAC address. Defined in RFC 826.
asymmetric
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.
asynchronous
The lack of an imposed time ordering on a bit stream. Practically,
both sides agree to the same speed, but there is no check or adjustment of the rates if
600 access interface
they are slightly different. However, because only 1 byte per transfer is sent, slight
differences in clock speed are not an issue.
ATM
Asynchronous Transfer Mode. The international standard for cell relay in
which multiple service types (such as voice, video, and data) are conveyed in fixedlength
(53-byte) cells. Fixed-length cells allow cell processing to occur in hardware,
thereby reducing transit delays.
authentication
In security, the verification of the identity of a person or a process.
See AAA.
authorization
In security, the determination of the rights allowed for a particular
user or device. See AAA.
autonomous system
An internetwork in the administrative control of one
organization, company, or governmental agency, inside which that organization
typically runs an Interior Gateway Protocol (IGP).
auxiliary port
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.
back-to-back link
A serial link between two routers, created without CSU/DSUs, by
connecting a DTE cable to one router and a DCE cable to the other. Typically used in
labs to build serial links without the expense of an actual leased line from the telco.
balanced hybrid
A term that refers to a general type of routing protocol algorithm,
the other two being distance vector and link state. The Enhanced Interior Gateway
Routing Protocol (EIGRP) is the only routing protocol that Cisco classifies as using a
balanced hybrid algorithm
bandwidth
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.
basic service set (BSS
In wireless LANs, a WLAN with a single access point.
bitwise Boolean AND 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.
ATM 601
Boolean AND
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.
boot field
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.
BRI
Basic Rate Interface. An ISDN interface composed of two 64-kbps bearer (B)
channels and one 16-kbps data (D) channel for circuit-switched communication of
voice, video, and data
broadcast address
See subnet broadcast address
broadcast domain
A set of all devices that receive broadcast frames originating
from any device within the set. Devices in the same VLAN are in the same broadcast
domain.
broadcast frame
An Ethernet frame sent to destination address FFFF.FFFF.FFFF,
meaning that the frame should be delivered to all hosts on that LAN.
broadcast subnet
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
bus
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.
CDP
Cisco Discovery Protocol. A media- and protocol-independent devicediscovery
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.
CDP neighbor
A device on the other end of some communications cable that is
advertising CDP updates
CIDR notation
See prefix notation.
602 Boolean AND
circuit switching
switching 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.
classful network
An IPv4 Class A, B, or C network; called a classful network
because these networks are defined by the class rules for IPv4 addressing.
classful routing protocol
Does not transmit the mask information along with the
subnet number, and therefore must consider Class A, B, and C network boundaries and
perform autosummarization at those boundaries. Does not support VLSM.
classless routing protocol
protocol 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.
CLI
Command-line interface. An interface that enables the user to interact with the
operating system by entering commands and optional arguments.
clock rate
The speed at which a serial link encodes bits on the transmission medium.
clock source
The device to which the other devices on the link adjust their speed
when using synchronous links.
clocking
clocking The process of supplying a signal over a cable, 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.
codec
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.
collision domain
A set of network interface cards (NICs) for which a frame sent by
one NIC could result in a collision with a frame sent by any other NIC in the same
collision domain.
configuration mode
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).
circuit switching 603
configuration register
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
connection establishmen
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
console port
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.
convergence
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
CPE
Customer premises equipment. Any equipment related to communications that
is located at the customer site, as opposed to inside the telephone company’s network.
CSMA/CA
CA Carrier sense multiple access with collision avoidance. A media-access
mechanism that defines how devices decide when they can send, with a goal of
avoiding collisions as much as possible. IEEE WLANs use CSMA/CA.
CSMA/CD
Carrier sense multiple access collision detect. 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.
CSU/DSU
Channel service unit/digital 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.
604 configuration register
cut-through switching
One of three options for internal processing on some models
of Cisco LAN switches in which the frame is forwarded as soon as possible, including
forwarding the first bits of the frame before the whole frame is received.
DCE
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.
decapsulation
On a computer that receives data over a network, the process in which
the device interprets the lower-layer headers and, when finished with each header,
removes the header, revealing the next-higher-layer PDU.
default gateway/default router
On an IP host, the IP address of some router to
which the host sends packets when the packet’s destination address is on a subnet other
than the local subnet.
default mask
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.
default route
On a router, the route that is considered to match all packets that are
not otherwise matched by some more specific route.
demarc
The legal term for the demarcation or separation point between the telco’s
equipment and the customer’s equipment.
denial of service (DoS)
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.
DHCP
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.
Direct Sequence Spread Spectrum (DSSS
A method of encoding data for
transmission over a wireless LAN in which the device uses 1 of 11 (in the USA) nearby
frequencies in the 2.4-GHz range.
distance vector
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, but only to its neighbors. Distance vector routing
algorithms can be prone to routing loops but are computationally simpler than linkstate
routing algorithms.
DNS
Domain Name System. An application layer protocol used throughout the
Internet for translating hostnames into their associated IP addresses.
DS0
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
DS1
Digital signal level 1. A 1.544-Mbps line from the telco, with 24 DS0 channels
of 64 kbps each, plus an 8-kbps management and framing channel. Also called a T1.
DSL
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.
DTE
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.
E1
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.
enable mode
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.
encapsulation
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 sent over an Ethernet.
encryption
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.
606 directed broadcast address
error detection
The process of discovering whether or not a data-link level frame
was changed during transmission. This process typically uses a Frame Check
Sequence (FCS) field in the data-link trailer.
error disabled
An interface state on LAN switches that is the result of one of many
security violations.
error recovery
The process of noticing when some transmitted data was not
successfully received and resending the data until it is successfully received.
Ethernet
A series of LAN standards defined by the IEEE, originally invented by
Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment
Corporation.
Extended Service Set (ESS)
) In wireless LANs, a WLAN with multiple access
points to create one WLAN, allowing roaming between the APs
Exterior Gateway Protocol (EGP)
A routing protocol that was designed to
exchange routing information between different autonomous systems
filter
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
firewall
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
Flash
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
flooding
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 forward multicasts by default,
although this behavior can be changed
flow control
The process of regulating the amount of data sent by a sending
computer toward a receiving computer. Several flow control mechanisms exist,
including TCP flow control, which uses windowing.
error detection 607
forward
To send a frame received in one interface out another interface, toward its
ultimate destination.
forward acknowledgment
t 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.
four-wire circuit
A line from the telco with four wires, composed of two twistedpair
wires. Each pair is used to send in one direction, so a four-wire circuit allows fullduplex
communication.
fragment-free switching
One of three internal processing options on some Cisco
LAN switches in which the first bits of the frame may 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.
frame
A term referring to a data-link header and trailer, plus the data encapsulated
between the header and trailer.
Frame Relay
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.
Frequency Hopping Spread Spectrum
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.
full duplex
Generically, any communication in which two communicating devices
can concurrently send and receive data. In Ethernet LANs, the allowance for both
devices to send and receive at the same time, allowed when both devices disable their
CSMA/CD logic.
full mesh
A network topology in which more than two devices can physically
communicate and, by choice, all pairs of devices are allowed to communicate directly
half duplex
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.
608 forward
HDLC
High-Level Data Link Control. A bit-oriented synchronous data link layer
protocol developed by the International Organization for Standardization (ISO).
head end The upstream, transmit end of a cable TV (CATV) installation
host
Any device that uses an IP address.
host address
The IP address assigned to a network card on a computer.
host part
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
host route
A route with a /32 mask, which by virtue of this mask represents a route
to a single host IP address.
HTTP
Hypertext Transfer Protocol. The protocol used by web browsers and web
servers to transfer files, such as text and graphic files.
hub
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
ICMP
Internet Control Message Protocol. A TCP/IP network layer protocol that
reports errors and provides other information relevant to IP packet processing.
IEEE
Institute of Electrical and Electronics Engineers. A professional organization
that develops communications and network standards, among other activities.
IEEE 802.2
An IEEE LAN protocol that specifies an implementation of the LLC
sublayer of the data link layer
IEEE 802.3
A set of IEEE LAN protocols that specifies the many variations of what
is known today as an Ethernet LAN.
HDLC 609
inactivity timer
For switch MAC address tables, a timer associated with each entry,
which counts time upwards 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.
infrastructure mode
A mode of wireless LAN (WLAN) operation in which WLAN
clients send and receive data with an access point (AP), which allows the clients to
communicate with the wired infrastructure through the AP. Clients do not send data to
each other directly; the AP must receive the data from one client, and then send the data
to the other WLAN client.
inside global
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.
inside local
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 Enterprise (private) part of the network.
Interior Gateway Protocol (IGP)
See interior routing protocol.
interior routing protocol
protocol A routing protocol designed for use within a single
organization.
inside global
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.
inside local
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 Enterprise (private) part of the network
intrusion detection system (IDS)
A security function that examines more complex
traffic patterns against a list of both known attack signatures and general characteristics
of how attacks may be carried out, rating each perceived threat and reporting the
threats.
intrusion prevention system (IPS)
A security function that examines more
complex traffic patterns against a list of both known attack signatures and general
characteristics of how attacks may be carried out, rating each perceived threat and
reacting to prevent the more significant threats.
IOS
Cisco operating system software that provides the majority of a router’s or
switch’s features, with the hardware providing the remaining features.
IOS Image
A file that contains the IOS.
IP
Internet Protocol. The network layer protocol in the TCP/IP stack, providing
routing and logical addressing standards and services.
IP address
In IP Version 4 (IPv4), a 32-bit address 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.
ISDN
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.
ISL
Inter-Switch Link. A Cisco-proprietary protocol that maintains VLAN
information as traffic flows between switches and routers.
ISO
International Organization for Standardization. An international organization
that is responsible for a wide range of standards, including many standards relevant to
networking. The ISO developed the OSI reference model, a popular networking
reference model.
keepalive
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
L4PDU
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.
Layer 3 protocol
A protocol that has characteristics like OSI Layer 3, which defines
logical addressing and routing. IP, IPX, and AppleTalk DDP are all Layer 3 protocols
learning
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.
leased line
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.
IP address 611
link state
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.
LLC Logical Link Control. The higher of the two data link layer sublayers defined
by the IEEE. Synonymous with IEEE 802.2.
local loop
A line from the premises of a telephone subscriber to the telephone
company CO.
logical address
A generic reference to addresses as defined by Layer 3 protocols,
which 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 physical addresses because they differ based on the type
of physical medium.
MAC
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
MAC address
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.
metric
A unit of measure used by routing protocol algorithms to determine the best
route for traffic to use to reach a particular destination
microsegmentation
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 fact that one
definition for the word “segment” is “collision domain,” with a switch separating each
switch port into a separate collision domain or segment.
modem
Modulator-demodulator. A device that converts between digital and analog
signals so that a computer may send data to another computer 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.
612 link state
multimode
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.
name server
A server connected to a network that resolves network names into
network addresses.
NAT
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.
network
A collection of computers, printers, routers, switches, and other devices
that can communicate with each other over some transmission medium.
network broadcast address
In IPv4, a special address in each classful network that
can be used to broadcast a packet to all hosts in that 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.
network number
A number that uses dotted decimal notation like IP addresses, but
the number itself represents all hosts in a single Class A, B, or C IP network
network part
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
networking model
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.
NVRAM
Nonvolatile RAM. A type of random-access memory (RAM) that retains
its contents when a unit is powered off.
multimode 613
ordered data transfer
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.
Orthogonal Frequency Division Multiplexing
A method of encoding data in
wireless LANs that allows for generally higher data rates than the earlier FHSS and
DSSS encoding methods.
OSI
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.
packet
A logical grouping of information that includes the network layer header and
encapsulated data, but specifically does not include any headers and trailers below the
network layer.
packet switching
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.
partial mesh
A network topology in which more than two devices could physically
communicate but, by choice, only a subset of the pairs of devices connected to the
network is allowed to communicate directly.
PCM
Pulse code modulation. A technique of encoding analog voice into a 64-kbps
data stream by sampling with 8-bit resolution at a rate of 8000 times per second.
PDU
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.
ping
Packet Internet groper. 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.
614 ordered data transfer
pinout
The documentation and implementation of which wires inside a cable
connect to each pin position in any connector
port
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.
Port Address Translation (PAT
A NAT feature in which one inside global IP
address supports over 65,000 concurrent TCP and UDP connections
port number
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.
positive acknowledgment and retransmission (PAR)
A generic reference to how
the error recovery feature works in many protocols, including TCP, in which the
receiver must send an acknowledgment that either implies that the data was (positively)
received, or send an acknowledgement that implies that some data was lost, so the
sender can then resend the lost data.
Power-on Self Test (POST)
The process on any computer, including routers and
switches, in which the computer hardware first runs diagnostics on the required
hardware before even trying to load a bootstrap program
PPP
Point-to-Point Protocol. A protocol that provides router-to-router and host-tonetwork
connections over synchronous point-to-point and asynchronous point-topoint
circuits
prefix notation
A shorter way to write a subnet mask in which the number of binary
1s in the mask is simply written in decimal. For instance, /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
PRI
Primary Rate Interface. An Integrated Services Digital Network (ISDN)
interface to primary rate access. Primary rate access consists of a single 64-kbps D
channel plus 23 (T1) or 30 (E1) B channels for voice or data
private addresses
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.
problem isolation
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.
protocol data unit (PDU)
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
Protocol Type field
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.
PSTN
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
PTT
Post, telephone, and telegraph. A government agency that provides telephone
services. PTTs exist in most areas outside of North America and provide both local and
long-distance telephone services
public IP address
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.
RAM
Random-access memory. A type of volatile memory that can be read and
written by a microprocessor.
RFC
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,
RIP
Routing Information Protocol. An Interior Gateway Protocol (IGP) that uses
distance vector logic and router hop count as the metric. RIP Version 1 (RIP-1) has
become unpopular, with RIP Version 2 (RIP-2) providing more features, including
support for VLSM.
RJ-45
A popular type of cabling connector used for Ethernet cabling. It is similar to
the RJ-11 connector used for telephone wiring in homes in the United States. RJ-45
allows the connection of eight wires
ROM
Read-only memory. A type of nonvolatile memory that can be read but not
written by the microprocessor.
ROMMON
A shorter name for ROM Monitor, which is a low-level operating
system that can be loaded into Cisco routers for several seldom needed maintenance
tasks, including password recovery and loading a new IOS when Flash memory has
been corrupted
root cause
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.
routed protocol
A protocol which defines packets that can be routed by a router.
Examples of routed protocols include AppleTalk, DECnet, and IP
Router Security Device Manager
The administrative web-based interface on a
router that allows for configuration and monitoring of the router, including the
configuration of DHCP and NAT/PAT.
routing protocol
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).
routing table
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
routing update
A generic reference to any routing protocol’s messages in which it
sends routing information to a neighbor.
running-config file
In Cisco IOS switches and routers, the name of the file that
resides in RAM memory, holding the device’s currently used configuration.
RxBoot
A limited-function version of IOS stored in ROM in some older models of
Cisco routers, for the purpose of performing some seldom needed low-level functions,
including loading a new IOS into Flash memory when Flash has been deleted or
corrupted.
same-layer interaction
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
Secure Shell (SSH)
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.
segment
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).
segmentation
The process of breaking a large piece of data from an application into
pieces appropriate in size to be sent through the network.
serial cable
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.
Service Set Identifier (SSID)
A text value used in wireless LANs to uniquely
identify a single WLAN.
setup mode
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
shared Ethernet
An Ethernet that uses a hub, or even the original coaxial cabling,
which results in the devices having to take turns sending data, sharing the available
bandwidth.
single-mode
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
sliding windows
For protocols such as TCP that allow the receiving device to dictate
the amount of data the sender can send before receiving an acknowledgment—a
concept called a window—a reference to the fact that the mechanism to grant future
windows is typically just a number that grows upwards slowly after each
acknowledgment, sliding upward.
SONET
Synchronous Optical Network. A standard format for transporting a wide
range of digital telecommunications services over optical fiber
Spanning Tree Protocol
A bridge 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 bridges to detect loops and then remove the loops by shutting down selected
bridge interfaces.
star
A network topology in which endpoints on a network are connected to a
common central device by point-to-point links
startup-config file
file 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 as the running-config file when the device is next reloaded or powered on
store-and-forward switching
One of three 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.
STP
Shielded twisted pair. Shielded twisted-pair cabling has a layer of shielded
insulation to reduce electromagnetic interference (EMI).
straight-through cable
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.
subnet
t 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.
subnet broadcast address
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.
subnet mask
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
subnet number
In IP v4, 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.
subnet part
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.
subnetting
The process of subdividing a Class A, B, or C network into smaller
groups called subnets.
switch
A network device that filters, forwards, and floods Ethernet frames based on
the destination address of each frame
switched Ethernet
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.
symmetric
A feature of many Internet access technologies in which the downstream
transmission rate is the same as the upstream transmission rate
synchronous
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.
T1
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
TCP
Transmission Control Protocol. A connection-oriented transport layer TCP/IP
protocol that provides reliable data transmission.
TCP/IP
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.
telco
A common abbreviation for telephone company.
Telnet
The standard terminal-emulation application layer protocol in the TCP/IP
protocol stack. Telnet is used for remote terminal connection, enabling users to log in
to remote systems and use resources as if they were connected to a local system. Telnet
is defined in RFC 854.
trace
Short for traceroute. A program available on many systems that traces the path
that a packet takes to a destination. It is used mostly to debug routing problems
between hosts.
transparent bridge
The name of a networking device that was a precursor to
modern LAN switches. Bridges forward frames between LAN segments based on the
destination MAC address. Transparent bridging is so named because the presence of
bridges is transparent to network end nodes.
trunk interface
On a LAN switch, an interface that is currently using either 802.1Q
or ISL trunking.
trunking
Also called VLAN trunking. A method (using either the Cisco ISL protocol
or the IEEE 802.1q protocol) to support multiple VLANs that have members on more
than one switch.
twisted pair
Transmission medium consisting of two insulated wires, with the wires
twisted around each other in a spiral. An electrical circuit flows over the wire pair, with
the current in opposite directions on each wire, which significantly reduces the
interference between the two wires.
UDP
User Datagram Protocol. Connectionless transport layer protocol in the
TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without
acknowledgments or guaranteed delivery.
unknown unicast frame
An Ethernet frame whose destination MAC address is not
listed in a switch’s MAC address table, so the switch must flood the frame.
up and up
Jargon referring to the two interface states on a Cisco IOS router or
switch (line status and protocol status), with the first “up” referring to the line status,
and the second “up” referring to the protocol status. An interface in this state should be
able to pass data-link frames.
update timer
A timer used by a router to indicate when to send the next routing
update.
URL
Universal Resource Locator. A standard for how to refer to any piece of
information retrievable via a TCP/IP network, most notably used to identify web
pages. For example, http://www.cisco.com/univercd is a URL that identifies the
protocol (HTTP), hostname (www.cisco.com), and web page (/univercd).
user mode
A mode of the user interface to a router or switch in which the user can
type only nondisruptive EXEC commands, generally just to look at the current status,
but not to change any operational settings.
UTP
Unshielded twisted pair. A type of cabling, standardized by the Electronics
Industry Alliance (EIA) and Telecommunications Industry Association (TIA), that
holds twisted pairs of copper wires (typically four pair), and does not contain any
shielding from outside interference.
variable-length subnet masks (VLSM
The capability to specify a different subnet
mask for the same Class A, B, or C network number on different subnets. VLSM can
help optimize available address space.
virtual circuit
In packet-switched services like Frame Relay, VC refers to the ability
of two DTE devices (typically routers) to send and receive data directly to each other,
which supplies the same function as a physical leased line (leased circuit), but doing
so without a physical circuit. This term is meant as a contrast with a leased line or
leased circuit.
virtual LAN (VLAN)
) A group of devices, connected to one or more switches, with
the devices grouped into a single broadcast domain through switch configuration.
VLANs allow switch administrators to separate the devices connected to the switches
into separate VLANs without requiring separate physical switches, gaining design
advantages of separating the traffic without the expense of buying additional hardware.
virtual private network (VPN)
The process of securing communication between
two devices whose packets pass over some public and unsecured network, typically the
Internet. VPNs encrypt packets so that the communication is private and authenticate
the identity of the endpoints.
VoIP
Voice over IP. The transport of voice traffic inside IP packets over an IP
network.
web server
Software, running on some computer, that stores web pages and sends
those web pages to web clients (web browsers) that request the web pages.
well-known port
A TCP or UDP port number reserved for use by a particular
application. The use of well-known ports allows a client to send a TCP or UDP
segment to a server, to the correct destination port for that application
Wi-Fi Alliance
An organization formed by many companies in the wireless industry
(an industry association) for the purpose of getting multivendor certified-compatible
wireless products to market in a more timely fashion than would be possible by simply
relying on standardization processes.
Wi-Fi Protected Access (WPA)
A trademarked name of the Wi-Fi Alliance that
represents a set of security specifications that predated the standardization of the IEEE
802.11i security standard.
window
The term window represents the number of bytes that can be sent without
receiving an acknowledgment.
wired equivalent privacy (WE P)
An early WLAN security specification that used
relatively weak security mechanisms, using only preshared keys and either no
encryption or weak encryption.
WLAN client
A wireless device that wants to gain access to a wireless access point
for the purpose of communicating with other wireless devices or other devices
connected to the wired internetwork
WPA2
The Wi-Fi Alliance trademarked name for the same set of security
specifications defined in the IEEE 802.11i security standard.
zero subnet
For every classful IPv4 network that is subnetted, the one subnet whose
subnet number has all binary 0s in the subnet part of the number. In decimal, the zero
subnet can be easily identified because it is the same number as the classful network
number.
crossover cable
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 3,6 to pins 1,2 as well