The Network Layer

Question 1

Define the network layer

Answer 1

The network layer, or OSI Layer 3, provides services to allow end devices to exchange data across networks

Question 2

What are the four basic operations performed by the network layer?

Answer 2

1) Addressing end devices
2) Encapsulation
3) Routing
4) De-encapsulation

Question 3

Talk about addressing end devices

Answer 3

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

Question 4

Talk about encapsulation

Answer 4

The network layer encapsulates the protocol data unit (PDU) from the transport layer into a packet. The encapsulation process adds IP header information (source and destination IP address)
The encapsulation process is performed by the source of the IP packet.

Question 5

Talk about routing

Answer 5

The network layer provides services to direct the packets to a destination host on another network. To travel to other networks, the packet must be processed by a router. The role of the router is to select the best path and direct packets toward the destination host in a process known as routing. A packet may cross many routers before reaching the destination host. Each router a packet crosses to reach the destination host is called a hop.

Question 6

Talk about De-encapsulation

Answer 6

When the packet arrives at the network layer of the destination host, the host checks the IP header of the packet. If the destination IP address within the header matches its own IP address, the IP header is removed from the packet. After the packet is de-encapsulated by the network layer, the resulting Layer 4 PDU is passed up to the appropriate service at the transport layer. The de-encapsulation process is performed by the destination host of the IP packet.

Question 7

What’s the function of network layer communication protocols?

Answer 7

IPv4 and IPv6 specify the packet structure and processing used to carry the data from one host to another host. Operating without regard to the data carried in each packet allows the network layer to carry packets for multiple types of communications between multiple hosts.

Question 8

What’s the PDU form for each layer?

Answer 8

Application/Session/Presentation: Data
Transport Layer: Segment
Network Layer: Packet
Data link Layer: Frame
Physical Layer: Bits

Question 9

Talk about IP Encapsulation

Answer 9

IP encapsulates the transport layer segment or other data by adding an IP header. The IP header is used to deliver the packet to the destination host.
The process of encapsulating data layer by layer enables the services at the different layers to develop and scale without affecting the other layers.
The IP header is examined by Layer 3 devices as it travels across a network to its destination. It is important to note, that the IP addressing information remains the same from the time the packet leaves the source host until it arrives at the destination host

Question 10

What are the characteristics of IP?

Answer 10

1) Connection-less
2) Best Effort
3) Media Independent
Note: IP was not designed to track and manage the flow of packets.

Question 11

What do we mean when we say IP is connection-less?

Answer 11

No dedicated end-to-end connection is created by IP before data is sent.
IP requires no initial exchange of control information to establish an end-to-end connection before packets are forwarded.

Question 12

What do we mean when we say IP is best effort?

Answer 12

IP is inherently unreliable because packet delivery is not guaranteed. (Unreliable means that IP does not have the capability to manage and recover from undelivered or corrupt packets. This is because while IP packets are sent with information about the location of delivery, they do not contain information that can be processed to inform the sender whether the delivery was successful).

Question 13

What’s the function of the IPv4 packet?

Answer 13

Used to ensure that this packet is delivered to its next stop on the way to its destination end device. It consists of fields containing important information about the packet. These fields contain binary numbers which are examined by the Layer 3 process.

Question 14

What are the significant fields in the IPv4 packet header?

Answer 14

1) Version
2) Differentiated service (DS)
3) Time to Live (TTL)
4) Protocol
5) Header Checksum
6) Source IPv4 address
7) Destination IPv4 address
The whole header is 20 bytes

Question 15

Talk about Version

Answer 15

Contains a 4-bit binary value that identifies this as an IPv4 packet

Question 16

Talk about Differentiated service (DS)

Answer 16

an 8-bit field used to determine the priority of each packet.

Question 17

Talk about Time to Live

Answer 17

an 8-bit binary value that is used to limit the lifetime of a packet. The source device of the IPv4 packet sets the initial TTL value. It is decreased by one each time the packet is processed by a router. If the TTL field decrements to 0, the router discards the packet.

Question 18

Talk about protocol

Answer 18

This 8-bit binary value is used to identify the next level protocol.

Question 19

Talk about Header Checksum

Answer 19

Used to detect the corruption in the IPv4 header.

Question 20

Talk about the source IPv4 address

Answer 20

Contains a 32-bit binary value that represents the source IPv4 address of the packet which is always a uni-cast.

Question 21

Talk about the destination IPv4 address

Answer 21

a 32-bit binary value that represents the destination IPv4 address of the packet which can be a uni-cast, multi-cast or broadcast.

Question 22

What’s another role of the network layer?

Answer 22

Direct packets between hosts

Question 23

A host can send packets to what?

Answer 23

1) Itself - A host can ping itself by sending a packet to a special IPv4 or IPv6 address
2) Local host - This is a destination host that is on the same local network as the sending host. The source and destination hosts share the same network address.
3) Remote host - This is a destination host on a remote network. The source and destination hosts do not share the same network address.

Question 24

Define a Default Gateway

Answer 24

The default gateway is the network device that can route traffic to other networks.

Question 25

On a network, a default gateway is usually a router with these features:

Answer 25

1) It has a local IP address in the same address range as other hosts on the local network.
2) It can accept data into the local network and forward data out of the local network.
3) It routes traffic to other networks.
Note: A default gateway is required to send traffic outside of the local network. Traffic cannot be forwarded outside the local network if there is no default gateway, the default gateway address is not configured, or the default gateway is down.

Question 26

Talk about when a host routes to the Default Gateway

Answer 26

A host routing table will typically include a default gateway. In IPv4, the host receives the IPv4 address of the default gateway either dynamically from Dynamic Host Configuration Protocol (DHCP) or configured manually, in IPv6, the router advertises the default gateway address or the host can be configured manually.
A default route is the route or pathway your computer will take when it tries to contact a remote network.

Question 27

Talk about host routing tables

Answer 27

On a Windows host, the route print or netstat -r command can be used to display the host routing table. Both commands generate the same output.
It displays three sections:
1) Interface List
2) IPv4 Route Table
3) IPv6 Route Table

Question 28

What does a host consult when it wants to send a packet to another host?

Answer 28

It’s routing table.
If the destination host is on a remote network, the packet is forwarded to the default gateway, which is usually the local router.

Question 29

What happens when a packet arrives on a router interface?

Answer 29

The router examines the destination IP address of the packet and searches its routing table to determine where to forward the packet. The routing table contains a list of all known network addresses (prefixes) and where to forward the packet. These entries are known as route entries or routes. The router will forward the packet using the best (longest) matching route entry.

Question 30

What are the three types of route entries that a routing table stores?

Answer 30

1) Directly-connected networks
2) Remote networks
3) Default route

Question 31

Talk about Directly-connected networks

Answer 31

These network route entries are active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each router interface is connected to a different network segment.

Question 32

Talk about remote networks

Answer 32

These network route entries are connected to other routers. Routers learn about remote networks either by being explicitly configured by an administrator or by exchanging route information using a dynamic routing protocol.

Question 33

Talk about Default route

Answer 33

Like a host, most routers also include a default route entry, a gateway of last resort. The default route is used when there is no better (longer) match in the IP routing table.

Question 34

A router can learn about remote networks in one of two ways:

Answer 34

1) Manually ( Static routing).
2) Dynamically - Remote routes are automatically learned using a dynamic routing protocol.

Question 35

Talk about static routing

Answer 35

Static routes are route entries that are manually configured. The static route includes the remote network address and the IP address of the next hop router.
If there is a change in the network topology, the static route is not automatically updated and must be manually reconfigured.

Question 36

Characteristics of static routing:

Answer 36

1) A static route must be configured manually.
2) The administrator needs to reconfigure a static route if there is a change in the topology and the static route is no longer viable.
3) A static route is appropriate for a small network and when there are few or no redundant links.

Question 37

What is the command used to view the IPv4 routing table?

Answer 37

show IP route privileged EXEC mode command

Question 38

Common route sources (codes) include these:

Answer 38

L - Directly connected local interface IP address
C – Directly connected network
S – Static route was manually configured by an administrator
O - OSPF
D - EIGRP