Layer 3: The Network Layer

The primary responsibility of the Network layer is to move data over an internetwork. This is called routing.

routing table

A table created by a router that contains information on how to reach networks that are directly attached to the router and networks that are distant.

hop

The number of foreign gateways (routers other than your own) that a packet must pass through between the source and destination computers.

At the Data-Link layer, addressing is physical. At the Network layer, addressing is logical. Each network must have a unique network address. This address routes packets to the correct network.

The main functions of the Network layer are to:

• Logically define networks based on unique network addresses.

• Determine how packets should be delivered based on current routing information. A route, or path, to a destination network can be created in several ways. The path can be manually entered by the network administrator; the path can be learned from other routers; or the path can be a default route, which means that the router relies on the next router to determine the best path.

• Provide network-level connection services.

Sending data through the Network layer is considered to be connectionless service. This is like sending a letter through the regular mail. You drop the mail in a postal box and assume that it will reach its destination. The Network layer is similar in that you send a packet and assume that it will reach its destination.

Network Layer Example

Referring to the example network shown below, assume that you want to send a packet from computer A to computer B. These steps will occur:

1. Each router contains routing tables that define the best path to all known networks. The first packet leaves computer A and is destined for computer B.

2. The packet first reaches the New York router. That router compares the destination address of the packet to its routing table.

3. The New York router determines that it must send the packet to the L.A. router to reach computer B. Each pass through an external router, not the New York router, is considered a hop.

4. The L.A. router determines the shortest path to computer B is through the Seattle router. The packet is sent on its way.

5. Seattle performs the same process as the preceding routers and sends the packet onto computer B's network, where computer B receives it.

   Note 

   Data is referred to as datagrams or packets at the Network layer.