Layer 3 - Network layer

The Network layer is concerned with finding the shortest path to the destination. This usually means finding the fastest route through multiple networks to the destination. Routing algorithms are used to determine the "shortest" path. Shortest does not mean the physically shortest distance but fastest route.

Figure 1

The Network layer converts the segments into smaller protocol data units (PDUs) called packets that the network can handle. The Network layer is connectionless in that it does not guarantee that the packet will reach its destination. It is often referred to as "send and pray". The packet is sent out on the wire and we pray that it arrives.

Networks are identified by network addresses that are separate from node addresses. Since the Network layer is concerned with finding networks, it adds the source and destination addresses to the packet.

The Network layer has several subsections:

IP Protocol - The Internet Protocol (IP) uses IP addresses and the subnet mask to determine whether the datagram is on the local or a remote network. If it is on the remote network, the datagram is forwarded to the default gateway which is a router that links to another network.

Address Resolution Protocol - The Address Resolution Protocol or ARP for short has the job of matching IP addresses to MAC frames. It is a protocol that is half in the Network Layer 3 and half in the Data Link Layer 2.

Subnetting - The classes of networks (A, B and C) do not provide much flexibility in designing a network. Each class of network only provides for a fixed number of networks (125, 16,382 or 2,097,150) and a fixed number of hosts (16,777,214, 65,534 or 254). Using the class system is referred to as having a classful network. In the real world, pretty much all networks do not fit the class system. The solution is to divide the class network into smaller subnetworks or subnets for short. The term for dividing networks into smaller subnets is called subnetting.

Routers - These are network devices that have the job of finding the shortest path to the destination network. The measurement of the shortest path is based on a metric that could be the number of transverses through routers called a hop count.

Brouters - Brouters are a combination of a bridge and a router. Sometimes acting as a router and sometimes as a bridge.

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Copyright July 2013 Eugene Blanchard