M1.8 Network Layer

Question 1

What is the primary function of the network layer or OSI Layer 3?

Answer 1

To provide services to allow end devices to exchange data across networks.

Question 2

What are the two principle network layer communication protocols?

Answer 2

IP version 4 (IPv4) and IP version 6 (IPv6).

Question 3

What are the four basic operations of network layer protocols?

Answer 3

Addressing end devices, Encapsulation, Routing and De-encapsulation.

Question 4

What is the purpose of ‘addressing end devices’ at the network layer?

Answer 4

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

Question 5

What is ‘encapsulation’ at the network layer?

Answer 5

The process of adding IP header information to the PDU from the transport layer.

Question 6

What is the network layer PDU called after encapsulation?

Answer 6

A packet.

Question 7

What is ‘routing’ at the network layer?

Answer 7

The process of selecting the best path and directing packets toward the destination host.

Question 8

What is a ‘hop’ in the context of routing?

Answer 8

Each router a packet crosses to reach the destination host.

Question 9

What is ‘de-encapsulation’ at the network layer?

Answer 9

The process of removing the IP header from the packet at the destination host.

Question 10

When does a host perform de-encapsulation?

Answer 10

When the destination IP address in the packet header matches its own IP address.

Question 11

What is a primary characteristic of the IP protocol regarding overhead?

Answer 11

IP was designed as a protocol with low overhead.

Question 12

What are the three basic characteristics of IP?

Answer 12

Connectionless Best Effort and Media Independent.

Question 13

What does the ‘connectionless’ characteristic of IP mean?

Answer 13

No dedicated end-to-end connection is established before sending data packets.

Question 14

What does the ‘best effort’ characteristic of IP mean?

Answer 14

Packet delivery is not guaranteed.

Question 15

What does ‘unreliable’ mean for the IP protocol?

Answer 15

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

Question 16

Which protocol in the TCP/IP suite is responsible for reliability?

Answer 16

The TCP protocol at the transport layer.

Question 17

What does the ‘media independent’ characteristic of IP mean?

Answer 17

IP’s operation is independent of the medium carrying the data like copper fiber-optic or wireless.

Question 18

What is the Maximum Transmission Unit (MTU)?

Answer 18

The maximum size of the PDU that each medium can transport.

Question 19

What is fragmentation?

Answer 19

The process of a router splitting an IPv4 packet to fit a smaller MTU.

Question 20

Which IP version’s packets cannot be fragmented by routers?

Answer 20

IPv6.

Question 21

What is the 4-bit binary value in the Version field that identifies an IPv4 packet?

Answer 21

0100

Question 22

What is the 8-bit DS field in an IPv4 header used for?

Answer 22

To determine the priority of each packet.

Question 23

What was the Differentiated Services (DS) field formerly called?

Answer 23

The type of service (ToS) field.

Question 24

The Differentiated Services (DS) field is composed of what two sub-fields?

Answer 24

The 6-bit Differentiated Services Code Point (DSCP) and the 2-bit Explicit Congestion Notification (ECN).

Question 25

What is the purpose of the Time to Live (TTL) field in an IPv4 header?

Answer 25

To limit the lifetime of a packet.

Question 26

What happens to the TTL value each time a packet is processed by a router?

Answer 26

It is decreased by one.

Question 27

What happens if a packet's TTL field decrements to zero?

Answer 27

The router discards the packet and sends an ICMP Time Exceeded message to the source IP address.

Question 28

What is the purpose of the Protocol field in an IPv4 header?

Answer 28

It identifies the data payload type the packet is carrying.

Question 29

What is the Protocol field value for ICMP?

Answer 29

1

Question 30

What is the Protocol field value for TCP?

Answer 30

6

Question 31

What is the Protocol field value for UDP?

Answer 31

17

Question 32

What is the purpose of the Header Checksum field in an IPv4 header?

Answer 32

To detect corruption in the IPv4 header.

Question 33

How many bits long is a source IPv4 address?

Answer 33

32 bits.

Question 34

How many bits long is a destination IPv4 address?

Answer 34

32 bits.

Question 35

What is the crucial difference between a source and destination address field in an IPv4 header?

Answer 35

The source address must be a unicast address while the destination can be unicast multicast or broadcast.

Question 36

Which three fields in an IPv4 header are used to reorder a fragmented packet?

Answer 36

1.Identification 2.Flags 3.Fragment Offset.

Question 37

What are three major issues of IPv4?

Answer 37

IPv4 address depletion lack of end-to-end connectivity and increased network complexity.

Question 38

What technology hides the IPv4 address of an internal network host?

Answer 38

Network Address Translation (NAT).

Question 39

How many bits are in an IPv6 address?

Answer 39

128 bits.

Question 40

What are two key improvements IPv6 provides?

Answer 40

Increased address space and improved packet handling.

Question 41

How does IPv6 improve packet handling compared to IPv4?

Answer 41

It uses a simplified fixed-length header with fewer fields for more efficient processing.

Question 42

What major IPv4 limitation is eliminated by IPv6's massive address space?

Answer 42

The need for Network Address Translation (NAT) to conserve public addresses.

Question 43

How large is the fixed-length IPv6 header?

Answer 43

40 octets.

Question 44

What is a key structural difference between the IPv4 header and the IPv6 header?

Answer 44

The IPv4 header is variable-length while the IPv6 header is a fixed length of 40 octets.

Question 45

What is the 4-bit binary value in the Version field that identifies an IPv6 packet?

Answer 45

0110

Question 46

Which 8-bit field in the IPv6 header is equivalent to the IPv4 DS field?

Answer 46

The Traffic Class field.

Question 47

What is the purpose of the 20-bit Flow Label field in the IPv6 header?

Answer 47

To suggest all packets with the same label receive the same handling by routers.

Question 48

Which 8-bit field in the IPv6 header is equivalent to the IPv4 Protocol field?

Answer 48

The Next Header field.

Question 49

Which 8-bit field in the IPv6 header replaces the IPv4 TTL field?

Answer 49

The Hop Limit field.

Question 50

Why does the IPv6 header not include a Header Checksum?

Answer 50

The function is performed at lower and upper layers improving performance.

Question 51

What are Extension Headers (EH) in an IPv6 packet?

Answer 51

Optional headers placed between the IPv6 header and the payload for optional information.

Question 52

How many unique addresses does the 32-bit IPv4 address space provide?

Answer 52

Approximately 4.3 billion.

Question 53

How many unique addresses does the 128-bit IPv6 address space provide?

Answer 53

340 undecillion.

Question 54

What three destination types can a host send a packet to?

Answer 54

Itself a local host or a remote host.

Question 55

What is the IPv4 loopback address used for a host to ping itself?

Answer 55

127.0.0.1.

Question 56

What is the IPv6 loopback address?

Answer 56

::1.

Question 57

What is a 'local host' in terms of routing?

Answer 57

A destination host on the same local network as the sending host.

Question 58

What is a 'remote host' in terms of routing?

Answer 58

A destination host on a remote network.

Question 59

How does an IPv4 host determine if a destination is local or remote?

Answer 59

It uses its own subnet mask its own IPv4 address and the destination IPv4 address.

Question 60

How does an IPv6 host determine if a destination is local?

Answer 60

The local router advertises the local network address (prefix).

Question 61

What is a default gateway?

Answer 61

A network device that can route traffic to other networks.

Question 62

What must be configured on a host to send traffic outside the local network?

Answer 62

A default gateway.

Question 63

How does an IPv4 host typically learn its default gateway address?

Answer 63

Dynamically from DHCP or it is configured manually.

Question 64

How does an IPv6 host typically learn its default gateway address?

Answer 64

The router advertises the address or it is configured manually.

Question 65

What command on a Windows host displays the routing table?

Answer 65

route print or netstat -r.

Question 66

What are the three sections of output from the 'netstat -r' command?

Answer 66

-Interface List -IPv4 Route Table and -IPv6 Route Table.

Question 67

What does a default gateway configuration create in a PC's routing table?

Answer 67

A default route.

Question 68

What does a router examine in a packet to make a forwarding decision?

Answer 68

The destination IP address.

Question 69

What are the three types of route entries in a router's routing table?

Answer 69

-Directly-connected networks -remote networks and a -default route.

Question 70

When does a router add a directly-connected route to its table?

Answer 70

When an interface is configured with an IP address and is activated.

Question 71

What are the two ways a router learns about remote networks?

Answer 71

Manually via static routes or dynamically via a routing protocol.

Question 72

When is a default route used by a router?

Answer 72

When there is no better or longer match in the IP routing table.

Question 73

What is a static route?

Answer 73

A route entry that is manually configured by an administrator.

Question 74

What is a major characteristic of static routing regarding network changes?

Answer 74

It is not automatically updated and must be manually reconfigured.

Question 75

What is dynamic routing?

Answer 75

A process that allows routers to automatically learn about remote networks from other routers.

Question 76

Name two dynamic routing protocols.

Answer 76

OSPF and EIGRP.

Question 77

What are the four key tasks a dynamic routing protocol performs automatically?

Answer 77

1.Discover remote networks 2.maintain up-to-date routing information 3.choose the best path and 4.find a new best path if the current one fails.

Question 78

What Cisco IOS command views the IPv4 routing table?

Answer 78

show ip route.

Question 79

In a Cisco routing table what does the code 'L' indicate?

Answer 79

A directly connected local interface IP address.

Question 80

In a Cisco routing table what does the code 'C' indicate?

Answer 80

A directly connected network.

Question 81

In a Cisco routing table what does the code 'S' indicate?

Answer 81

A static route was manually configured.

Question 82

In a Cisco routing table what does the code 'O' indicate?

Answer 82

The route was learned via OSPF.

Question 83

In a Cisco routing table what does the code 'D' indicate?

Answer 83

The route was learned dynamically via EIGRP.

Question 84

What three steps does a router take to forward a packet?

Answer 84

1. De-encapsulates the Layer 2 frame 2. Examines the destination IP to find the best route 3. Encapsulates the packet in a new Layer 2 frame and forwards it.

Question 85

Name the IPv4 header fields

Answer 85

1. Version 2. Differentiated Servcies (DS) 3. Time to Live (TTL) 4. Protocol 5. Header Checksum 6. Source IPv4 Address 7. Destination IPv4Address

Question 86

Name the IPv6 header fields

Answer 86

1. Version 2. Traffic Class 3. Flow Label 4. Payload length 5. Next Header 6. Hop Limit 7. Source IPv6 Address 8. Destination IPv6 Address