Loopback Interfaces

Router(config)#interface lo0	Moves to virtual interface Loopback 0
Router(config-if)#ip address 192.168.100.1 255.255.255.255	Assigns IP address to interface

Note:

Loopback interfaces are always "up and up" and do not go down unless manually shut down. This makes Loopback interfaces great for using as an OSPF router ID.