IP Addressing on a Router

The role of a router is to receive IP packets and route them from one subnet to another based on routing rules that exist within the router (statically assigned routes or routes learned from routing protocols such as OSPF or EIGRP).

As such, a router will have multiple layer 3 interfaces, each one assigned an IP address on a different subnet. It will receive a packet on one interface and route it out another.

Router interfaces cannot be assigned IP addresses in the same subnet. If you attempt to configure two interfaces on the same subnet, it will give you an error. Such a configuration defeats the purpose of the router, which is to route packets from one subnet to another.

This differs somewhat from IP addressing on a Switch.