6. Network Layer

Front 1

What do protocols of the OSI model network layer specify?

Back 1

Addressing and processes that enable transport layer data to be packaged and transported.

Front 2

What does the network layer encapsulation enable?

Back 2

Data to be passed to a destination within a network (or on another network) with minimum overhead.

Front 3

What is the communication between networks called?

Back 3

Routing.

Front 4

What does the network layer provide?

Back 4

OSI layer 3 provides services to allow end devices to exchange data across the network.

Front 5

What are the four basic processes that the network layer uses to accomplish end-to-end transport?

Back 5

-Addressing end devices

-Encapsulation

-Routing

-De-encapsulation

Front 6

What is "Addressing end devices"?

Back 6

End devices must be configued with a unique IP address for identification on the network.

Front 7

What is a host?

Back 7

It is an end device with a configured IP address.

Front 8

What is "Encapsulation"?

Back 8

The network layer receives a PDU from the transport layer. The network layer adds IP header information, e.g. IP address of the source and destination hosts. The PDU is now a packet.

Front 9

What is "Routing"?

Back 9

To travel to other networks, the packet must be processed by a router. The router will then select paths for and direct packets toward the destination host in a process known as routing. Many intemediary devices may be passed before getting to the destination host.

Front 10

What is a hop?

Back 10

Each route the packet takes to reach the destination host.

Front 11

What is "De-encapsulation"?

Back 11

When the packet is at the network layer of the destination host, the host checks the IP header of the packet. If there is a match with their own IP address then the IP header is removed from the packet. This process is known as de-encapsulation (removing headers from lower layers).

Front 12

What happens after the packet is de-encapsulated?

Back 12

The resulting Layer 4 PDU is passed up to the appropriate service at the transport layer.

Front 13

What does the transport layer (OSI Layer 4) do?

Back 13

It manages the data transport between the processes running on each host.

Front 14

What do network layer protocols specify that the transport layer (OSI Layer 4) doesn't specify?

Back 14

The packet structure and processing used to carry the data from one host to another host.

Front 15

What are the two commonly implemented network layer protocols?

Back 15

-Internet Protocol version 4 (IPv4)

-Internet Protocol version 6 (IPv6)

Front 16

What are other legacy network layer protocols that aren't widely known?

Back 16

-Novell Internetwork Packet Exchange (IPX)

-AppleTalk

Connectionless Network Service (CLNS/DECNet)

Front 17

What is the network layer service implemented by the TCP/IP protocol suite?

Back 17

IP

Front 18

What is IP?

Back 18

It was designed as a protocol with low overhead. It provides only the functions that are necessary to deliver a packet from a source to a destination over an interconnected system of networks. It was not designed to track and manager the flow of packets.

Front 19

What are the basic characteristics of IP?

Back 19

-Connectionless = No connection with destination is established before sending data packets.

-Best effort(unreliable) = Packet delivery is not guaranteed.

-Media Independent = Operation is independent of the medium carrying the data.

Front 20

What is the role of the network layer?

Back 20

To transport packets between hosts while placing as little burden on the network as possible.

Front 21

What is the network layer not concerned with or even aware of?

Back 21

The type of communication contained inside of a packet. IP is connectionless, meaning that no dedicated end-to-end connection is created before data is sent.

Front 22

What is IP often referred to as?

Back 22

Unreliable or best-effort delivery protocol.

Front 23

What does unreliable mean?

Back 23

That IP does not have the capability to manage and recover from undelivered or corrupt packets.

Front 24

Why is IP unreliable?

Back 24

While IP packets are sent with information about the location of delivery, it contains no information that can be processed to inform the sender whether delivery was successful.

Front 25

Where does the reliability lie?

Back 25

The decision of reliability is to the transport layer making IP more adaptable and accomodating for different types of communication.

Front 26

What is an example of a connection-oriented protocol and what does it require?

Back 26

TCP, it requires that control data be exchanged to establish the connection. To maintain information about the connection, TCP also requires additional fields in the PDU header.

Front 27

What does IP operate independently of?

Back 27

The media that carry the data at lower layers of the protocol stack.

Front 28

What is the OSI data link layer's responsibility?

Back 28

To take an IP packet and prepare it for transmission over the communications medium. This means that the transport of IP packets is not limited to any particular medium.

Front 29

What is one major characteristic of the media that the network layer considers?

Back 29

The maximum size of the PDU that each medium can transport. This characteristic is referred to as the maximum transmission unit (MTU).

Front 30

What is fragmentation?

Back 30

a.k.a fragmenting the packet, in some cases, an intermediate device, usually a router will need to split up a packet when forwarding it from one medium to a medium with a smaller MTU.

Front 31

What does the process of encapsulating data layer by layer enable?

Back 31

The services at the different layers to develop and scale without affecting other layers, meaning that the transport layer segments can be readily packaged by any new protocol that might be developed in the future (like IPv6).

Front 32

What remains unchanged during the network layer processes?

Back 32

In all cases, the data portion of the pcaket, that is the encapsulated transport layer PDU.

Front 33

What are the two parts of an IPv4 packet?

Back 33

-IP Header = Identifies the packet characteristics.

-Payload = Contains the Layer 4 segment information and the actual data.

Front 34

What does an IPv4 packet look like?

Back 34

Front 35

What are the significant fields in an IPv4 header?

Back 35

-Version

-Differentiated Services (DS)

-Time-to-Live (TTL)

-Protocol

-Source IP Address

-Destination IP Address

Front 36

Explain the Version field in an IPv4 header.

Back 36

Contains a 4-bit binary value identifying the IP packet version. For IPv4 packets, this field is always set to 0100.

Front 37

Explain the DS field in an IPv4 header.

Back 37

Formerly called the Type of Service (ToS) field, it is an 8-bit field used to determine the priority of each packet. The first 6 bits identify the Differentiated Services Code Point (DSCP) value that is used by a quality of service (QoS) mechanism. The last 2 bits identify the explicit congestion notification (ECN) value that can be used to prevent dropped packets during times of network congestion.

Front 38

Explain the TTL field in an IPv4 header.

Back 38

Contains an 8-bit binary value that is used to limit the lifetime of a packet. It is specified in seconds but is commonly referred to as hop count.

Front 39

How does the TTL process work?

Back 39

The packet sender sets the initial time-to-live (TTL) value and is decreased by one each time the packet is processed by a router, or hop. If the TTL field decrements to zero, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time Exceeded message to the source IP address.

Front 40

What is the traceroute command and what field does it use?

Back 40

It uses the TTL field to identify the routers used between the source and destination.

Front 41

Explain the Protocol field in an IPv4 header.

Back 41

This 8-bit binary value indicates the data payload type that the packet is carrying, which enables the network layer to pass the data to the appropriate upper-layer protocol. Common values include ICMP (1), TCP (6), and UDP (17).

Front 42

Explain the Source IP Address field in an IPv4 header.

Back 42

Contains a 32-bit binary value that represents the source IP address of the packet.

Front 43

Explain the Destination IP Address field in an IPv4 header.

Back 43

Contains a 32-bit binary value that represents the destination IP address of the packet.

Front 44

Which fields are used to identify and validate the IP packet?

Back 44

-Internet Header Length (IHL)

-Total Length

-Header Checksum

Front 45

Explain the IHL field.

Back 45

Contains a 4-bit binary value identifying the number of 32-bit words in the header. The IHL value varies due to the Options and Padding fields. The minimum value for this field is 5 and the maximum value is 15.

Front 46

Explain the Total Length field.

Back 46

Sometimes referred to as the Packet Length, this 16-bit field defines the entire packet (fragment) size, including header and data, in bytes. The minimum length packet is 20 bytes and maximum length is 65,535 bytes.

Front 47

Explain the Header Checksum field.

Back 47

The 16-bit field is used for error checking of the IP header. The checksum of the header is recalculated and compared to the value in the checksum field. If the values do not match, the packet is discarded.

Front 48

What fields are used to keep track of fragments (during a fragmentation process)?

Back 48

-Identification

-Flags

-Fragment Offset

Front 49

Explain the Identification field.

Back 49

This 16-bit field uniquely identifies the fragment of an original IP packet.

Front 50

Explain the Flags field.

Back 50

This 3-bit field identifies how the packet is fragmented. It is used with the Fragment Offset and Identification fields to help reconstruct the fragment into the original packet.

Front 51

Explain the Fragment Offset field.

Back 51

This 13-bit field identifies the order in which to place the packet fragment in the reconstruction of the original unfragmented packet.

Front 52

What is Wireshark?

Back 52

A useful network monitoring tool for anyone working with networks and can be used with most labs in the Cisco Certified Network Associate (CCNA) courses for data analysis and troubleshooting. It can be used to view sample values contained in IP header fields.

Front 53

What are the three major issues of IPv4?

Back 53

-IP address depletion

-Internet routing table expansion

-Lack of end-to-end connectivity

Front 54

Explain the issue: IP address depletion.

Back 54

IPv4 has a limited number of unique public IP addresses available. Although there are approximately 4 billion IPv4 addresses, the increasing number of new IP-enabled devices, always-on connections, and the potential growth of less-developed regions have increased the need for more addresses.

Front 55

Explain the issue: Internet routing table expansion.

Back 55

A routing table is used by routers to make best path determinations. As the number of servers (nodes) connected to the Internet increases, so too does the number of network routes. These IPv4 routes consume a great deal of memory and processor resources on Internet routers.

Front 56

What is a technology commonly implemented within IPv4 networks?

Back 56

Network Address Translation (NAT).

Front 57

What is NAT?

Back 57

NAT provides a way for multiple devices to share a single public IP address. However, because the public IP address is shared, the IP address of an internal network host is hidden. This can be problematic for technologies that require end-to-end connectivity.

Front 58

What led to the development of IPv6?

Back 58

When the Internet Engineering Task Force (IETF) grew concerned about the issues with IPv4 they began to look for a replacement.

Front 59

What improvements does IPv6 provide?

Back 59

-Increased address space

-Improved packet handling

-Eliminates the need for NAT

-Integrated security

Front 60

How does IPv6 provide increased address space?

Back 60

They are based on 128-bit hierarchical addressing as opposed to IPv4 with 32 bits (more bits) which increases the number of available IP addresses.

Front 61

How does IPv6 provide improved packet handling?

Back 61

The header has been simplified with fewer fields, improving packet handling by intermediate routers and also provides support for extensions and options for increased scalability/longevity.

Front 62

How does IPv6 provide elimination for the need for NAT?

Back 62

No need for NAT as there are loads of public IPv6 addresses.

Front 63

How does IPv6 provide integrated security?

Back 63

It natively supports authentication and privacy capabilities (additional features had to be implemented on IPv4 to do this).

Front 64

How many octets and basic header fields does the IPv4 header consist of?

Back 64

20 octets and 12 basic header field (exc. options field and padding field)

Front 65

How many octets and basic header fields does the IPv6 header consist of?

Back 65

40 octets and 8 header fields (3 basic IPv4 ones and 5 additional header fields).

Front 66

What does an IPv6 packet look like compared to an IPv4 packet?

Back 66

Front 67

What are some advantages for an IPv6 simplified header over an IPv4 one?

Back 67

-Better routing efficiency for performance and forwarding-rate scalability

-No requirement for processing checksums

-Simplified and more efficient extension header mechanisms

-A flow label field for per-flow processing with no need to open the transport inner packet to identify various traffic flow.

Front 68

What are the fields of an IPv6 packet header?

Back 68

-Version

-Traffic Class

-Flow Label

-Payload Length

-Next Header

-Hop Limit

-Source Address

-Destination Address

Front 69

Explain the Version field in an IPv6 packet header.

Back 69

This field contains a 4-bit binary value identifying the IP packet version. For IPv6 packets, this field is always set to 0110.

Front 70

Explain the Traffic Class field in an IPv6 packet header.

Back 70

This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field. It also contains a 6-bit Differentiated Services Code Point (DSCP) value used to classify packets and a 2-bit Explicit Congestion Notification (ECN) used for traffic congestion control.

Front 71

Explain the Flow Label field in an IPv6 packet header.

Back 71

This 20-bit field provides a special service for real-time applications. It can be used to inform routers and switches to maintain the same path for the packet flow so that packets are not reordered.

Front 72

Explain the Payload Length field in an IPv6 packet header.

Back 72

This 16-bit field is equivalent to the Total Length field in the IPv4 header. It defines the entire packet (fragment) size, including header and optional extensions.

Front 73

Explain the Next Header field in an IPv6 packet header.

Back 73

This 8-bit field is equivalent to the IPv4 Protocol field. It indicates the data payload type that the packet is carrying, enabling the network layer to pass the data to the appropriate upper-layer protocol. This field is also used if there are optional extension headers added to the IPv6 packet.

Front 74

Explain the Hop Limit field in an IPv6 packet header.

Back 74

This 8-bit field replaces the IPv4 TTL field. This value is decremented by one by each router that forwards the packet. When the counter reaches 0 the packet is discarded and an ICMPv6 message is forwarded to the sending host, indicating that the packet did not reach its destination.

Front 75

Explain the Source Address field in an IPv6 packet header.

Back 75

This 128-bit field identifies the IPv6 address of the sending host.

Front 76

Explain the Destination Address field in an IPv6 packet header.

Back 76

This 128-bit field identifies the IPv6 address of the receiving host.

Front 77

What are extension headers?

Back 77

They provide optional network layer information. Extension headers are optional and are placed between the IPv6 header and the payload. EHs are used for fragmentation, security, to support mobility, and more.

Front 78

What does an IPv6 packet header look like?

Back 78

Front 79

What is another role of the network layer?

Back 79

To direct packets between hosts.

Front 80

Where can a host send a packet to?

Back 80

-Itself

-Local host

-Remote host

Front 81

How does a host send a packet to itself?

Back 81

It can ping itself by sending the packet to a special IPv4 address of 127.0.0.1 which is referred to as the loopback interface.

Front 82

When is a loopback address automatically assigned?

Back 82

When TCP/IP is running.

Front 83

What is the IP within the network that refers to the local host?

Back 83

127.0.0.0/8

Front 84

What is a local host?

Back 84

A host on the same network as the sending host, they share the same network address.

Front 85

What is a remote host?

Back 85

A host on a remote network to the sending host, they do not share the same network address.

Front 86

What is the router connected to the local network segment referred to as?

Back 86

The default gateway.

Front 87

What is the default gateway?

Back 87

A device that routes traffic from the local network to devices on remote networks, it is often used to connect a local network to the Internet.

Front 88

What is a routing table?

Back 88

A data file in RAM that is used to store route information about directly connected network, as well as entries of remote networks the device has learned about.

Front 89

What does the local table of the host typically contain?

Back 89

-Direct connection (route to loopback interface = 127.0.0.1)

-Local network route = Network which the host is connected to is automatically populated in the host routing table.

-Local default route = Represents route that packets must take to reach all remote network addresses, created when default gateway address is present on the host.

Front 90

What is the default gateway address?

Back 90

The IP address of the network interface of the router that is connected to the local network, it can be configured manually or learned dynamically.

Front 91

What is important to note about a default gateway?

Back 91

The default route, and therefore, the default gateway, is only used when a host must forward packets to a remote network. It is not required, nor even needs to be configured, if only sending packets to devices on the local network.

Front 92

What does the route print/netstat -r command do?

Back 92

On a Windows host, it can be used to display the host routing table, both commands = same output.

Front 93

What three sections are displayed with the netstat -r/route print command?

Back 93

Related to the current TCP/IP network connections:

-Interface List = Lists MAC address and assigned interface number of every network-capable interface on the host including Ethernet, Wi-Fi, and Bluetooth adapters.

-IPv4 Route Table = Lists all known IPv4 routes, including direct connections, local network, and local default routes.

-IPv6 Route Table = Lists all known IPv6 routes, including direct connections, local network, and local default routes.

Front 94

What are the five sections in the IPv4 Route Table?

Back 94

-Network Destination = Lists the reachable networks.

-Netmask = Lists a subnet mask that informs the host how to determine the network and the host portions of the IP address.

-Gateway = Lists the addresses used by the local computer to get to a remote network destination.

-Interface = Lists the addresses of the physical interface used to send the packet to the gateway that is used to reach the network destination.

-Metric =Lists the cost of each route and is used to determine the best route to a destination.

Front 95

What is 127.0.0.0 - 127.255.255.255?

Back 95

These loopback addresses all relate to the direct connection and provide services to the local host.

Front 96

What is 192.168.10.0 - 192.168.10.255?

Back 96

These are addresses that all relate to the host and local network, all packets with destination addresses that fall into this category will exit out of the 192.168.10.10 interface.

Front 97

What is 192.168.10.0?

Back 97

The local network route address; represents all computers on the 192.168.10.x network.

Front 98

What is 192.168.10.10?

Back 98

The address of the local host.

Front 99

What is 192.168.10.255 ?

Back 99

The network broadcast address; sends messages to all hosts on the local network.

Front 100

What is 224.0.0.0?

Back 100

Special multicast class D addresses reserved for use through either the loopback interface (127.0.0.1) or the host IP address.

Front 101

What is 255.255.255.255 ?

Back 101

Represents the limited broadcast IP address values for use through either the loopback interface or the host IP address. These addresses can be used to find a DHCP server before the local IP is determined.

Front 102

What are the four sections in the IPv6 Route Table?

Back 102

-If = Lists the interface numbers from the Interface List section of the netstat –r command. The interface numbers correspond to the network capable interface on the host, including Ethernet, Wi-Fi, and Bluetooth adapters.

-Metric = Lists the cost of each route to a destination. Lower numbers indicate preferred routes.

-Network Destination = Lists the reachable networks.

-Gateway = Lists the address used by the local host to forward packets to a remote network destination. On-link indicates that the host is currently connected to it.

Front 103

What is to be noted about interfaces in IPv6?

Back 103

They commonly have two IPv6 addresses: link local address and a global unicast address. (Also notice there are no broadcast addresses in IPv6)

Front 104

What is ::/0?

Back 104

This is the IPv6 equivalent of the local default route.

Front 105

What is ::1/128?

Back 105

This is equivalent to the IPv4 loopback address and provides services to the local host.

Front 106

What is ::2001/32?

Back 106

This is the global unicast network prefix.

Front 107

What is 2001:0:9d38:953c:2c30:3071:e718:a926/128?

Back 107

This is the global unicast IPv6 address of the local computer.

Front 108

What is fe80::/64?

Back 108

This is the local link network route address and represents all computers on the local link IPv6 network.

Front 109

What isfe80::2c30:3071:e718:a926/128?

Back 109

This is the link local IPv6 address of the local computer.

Front 110

What is ff00::/8?

Back 110

These are special reserved multicast class D addresses equivalent to the IPv4 224.x.x.x addresses.

Front 111

What happens when a packet arrives on a router interface?

Back 111

The router looks at its routing table to determine where to forward packets.

Front 112

What information does a routing table of a router store?

Back 112

-Directly-connected routes

-Remote routes

Front 113

What are directly-connected routes?

Back 113

These routes come from the active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each of the router's interfaces is conneted to a different network segment. Routers maintain information about the network segments that they are connected to within the routing table.

Front 114

What are remote routes?

Back 114

These routes come from remote network connected to other routers. Routes to these networks can either be manually configured on the local router by the network administrator or dynamically configured by enabling the local router to exchange routing information with other routers using dynamic routing protocols.

Front 115

What does a host routing table include?

Back 115

Only information about directly-connected networks.

Front 116

What do both a routing table and a host routing table identify?

Back 116

-Destination network

-Metric associated with the destination network

-Gateway to get to the destination network

Front 117

What does the show ip route command do?

Back 117

Displayes the routing table of a router.

Front 118

What additional route information can a router also provide?

Back 118

How the route was learned, when it was last updated, and which specific interface to use to get to a predefined destination.

Front 119

What happens when a packet arrives at the router interface?

Back 119

The router examines the packet header to determine the destination network. If the destination network matches a route in the routing table, the router forwards the packet using the information specified in the routing table. If there are two or more possible routes to the same destination, the metric is used to decide which route appears on the routing table.

Front 120

What happens when an active router interface is configured with an IP address and subnet mask?

Back 120

Two routing table entries are automatically created.

Front 121

What information is contained the entries of routing tables?

Back 121

-Route Source

-Destination network

-Outgoing interface

Front 122

What is a route source?

Back 122

It identifies how the route was learned. Directly connected interfaces have two route source codes.

Front 123

What are the two route source codes?

Back 123

-C = Identifies a directly connected network. Directly connected networks are automatically created when an interface is configured with an IP address and activated.

- L =Identifies that this is a link local route. Link local routes are automatically created when an interface is configured with an IP address and activated.

Front 124

What is the destination network?

Back 124

It identifies the address of the remote network.

Front 125

What is the outgoing interface?

Back 125

It identifies the exit interface to use when forwarding packets to the destination network.

Front 126

What are common codes for remote networks?

Back 126

-S = Identifies that the route was manually created by an administrator to reach a specific network. This is known as a static route.

-D =Identifies that the route was learned dynamically from another router using the Enhanced Interior Gateway Routing Protocol (EIGRP).

-O =Identifies that the route was learned dynamically from another router using the Open Shortest Path First (OSPF) routing protocol.

Front 127

What information is included in routing table entries?

Back 127

-Route source

-Destination network

-Administrative distance

-Metric

-Next-hop

-Route timestamp

-Outgoing interface

Front 128

What is Administrative distance?

Back 128

Identifies the trustworthiness of the route source.

Front 129

What is Metric?

Back 129

Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.

Front 130

What is next-hop?

Back 130

Identifies the IP address of the next router to forward the packet.

Front 131

What is route timestamp?

Back 131

Identifies when the route was last heard from.

Front 132

What is the next hop for a packet that must be sent to another network?

Back 132

It must be the default gateway (router interface).

Front 133

Why do networks directly connected to a router have no next-hop address?

Back 133

Because a router can forward packets directly to hosts on these networks using the designated interface.

Front 134

When is a packet dropped?

Back 134

If a route representing the destination network is not in the routing table, the packet is dropped (that is, not forwarded).

Front 135

What are the four types of memory that a router has access to?

Back 135

-RAM

-ROM

-NVRAM

-Flash

Front 136

What is RAM used to store?

Back 136

-Cisco IOS

-Running configuration file

-IP routing table

-ARP cache

-Packet buffer

Front 137

What is DRAM?

Back 137

A very common kind of RAM that stores the instructionsand data needed to be executed by the CPU.

Front 138

What do Cisco routers use ROM to store?

Back 138

-Bootup instructions

-Basic diagnostic software

-Limited IOS

Front 139

What is ROM?

Back 139

It is firmware embedded on an integrated circuit inside the router and does not lose its contents when the router loses power or is restarted.

Front 140

What is NVRAM?

Back 140

It is used by the Cisco IOS as permanent storage for the startup configuration file (startup-config). Like ROM, it does not lose its contents when power is turned off.

Front 141

What is Flash memory?

Back 141

It is non-volatile computer memory used as permanent storage for the IOS and other system related files. The IOS is copied from flash into RAM during the bootup process.

Front 142

What are the hardware components of a router?

Back 142

-Power supply

-Shield for WAN interface card, WIC or high-speed WIC

-Fan

-SDRAM

-NVRAM

-CPU

-AIM

Front 143

What are the connections of a router?

Back 143

-Console ports

-AUX port

-Two LAN interfaces

-Enhanced high-speed WAN interface card

-Storage slots