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Routing comes into play when a computer on one network needs to send a packet to a computer on another network. In this case, a router is used to forward the packet to the destination network. In some cases, a packet may have to travel through several intermediate networks in order to reach its final destination network. You can find out more about routers in Chapter 4 of this minibook.

An important feature of routers is that you can use them to connect networks that use different layer 2 protocols. For example, a router can be used to send a packet from an Ethernet to a token ring network. As long as both networks support the same layer 3 protocol, it doesn’t matter whether their layer 1 and layer 2 protocols are different.

A protocol is considered routable if it uses addresses that include a network part and a host part. Any protocol that uses physical addresses isn’t routable because physical addresses don’t indicate to which network a device belongs.