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Recalling that a subnet consists of connected nodes running the same Data Link layer protocol, the Network layer's primary function is to enable data transfer between nodes that may or may not be in the same subnet. Hence, Network layer protocols must ensure that two things happen:

 Nodes in different subnets will communicate using a gateway/router.

 Nodes in the same subnet will communicate with one another using the Data Link layer protocol.

It may seem redundant for the Network layer to enable connectivity between nodes in the same subnet, since the Data Link layer already provides this functionality. But the purpose of the Network layer is to abstract the physical and data link characteristics of the network away so that applications don't need to be concerned with them. Instead, the application just deals with Network layer addresses—usually IP addresses.

To see how IP abstracts away the Data Link layer, compare the layer 2 and layer 3 topologies shown in Figure 1.5.

Figure 1.5 Simple layer 2 and layer 3 topologies

IP creates an addressing scheme on top of the Data Link layer, giving each subnet a different CIDR—a combination of an IP subnet address and subnet mask:

 VLAN 700—192.168.70.0/26

 PPP—10.255.70.0/31

 VLAN 706—192.168.70.64/26

A CIDR is the name that IP uses to address a subnet. Hence, a CIDR and subnet should always be tightly coupled, but they're not the same thing. The purpose of a CIDR (IP subnet address and mask) is to help a node determine based on the destination's IP address whether it's in the same subnet or a different subnet. If the destination's IP is in the same CIDR, the node assumes it's in the same subnet and will address the frame to the node's MAC address. Otherwise, the node will assume the destination is in another subnet and will address the frame to the MAC address of the default gateway for the subnet.

The OSI's dream of turning the network into a software abstraction begins to show cracks in the Network layer. Applications do indeed need to have some knowledge of the network, even if it's just IP addresses.