Cisco routers have long offered different ways of addressing their interfaces. In this tip, we'll look at the differences, and when you might or might not want to use these.

The first method is the one with which everyone is familiar. Simply using the ip address command from the interface configuration prompt assigns a single IP address to the interfaces, and the mask parameter lets the router know what the subnet is.

The second method, often called multinetting is when you put two or more IP subnets in the same broadcast domain. This is accomplished by using the same ip address command, but adding the secondary keyword at the end of the second and following statements. (i.e. if you have 3 subnets on the same broadcast domain, you still use secondary at the end of the third statement, and not third or tertiary)

The last method we'll discuss is the subinterface. Long ago, this was useful for Frame-relay and ATM PVCs on the WAN, where the layer 2 technology provided virtual circuits that needed to be in separate IP subnets, but were physically connected to a single interface. Later, this became popular on LAN interfaces with the advent of VLAN trunking. It is the same concept. These are usually configured like regular interfaces, but you go into the interface by using a "." and choosing a number. For instance, interface Serial0.1. In VLAN trunking, the number after the "." corresponds