Every active interface on a Cisco router to be used with IP requires an IP address be assigned to it. This short tech-recipe describes how to set an interface’s IP address.
Given an interface fastethernet1, the IP address 192.168.1.5 with the subnet mask 255.255.255.0 can be assigned to it using the following command from privileged mode:
Router(config-if)#ip address 192.168.1.5 255.255.255.0
The last command is important. It is an abbreviated form of “no shutdown.” The default state of an interface is “shutdown” which makes the interface administratively down. If you plumb up an interface and it does not work, check to make sure it is not administratively down first.