Please see the below pic. there are two Router R2 and R3.
I set the IP on R2 and R3 interface is up but no IP address.
R2 Interface Status and PORT. is up
R2#show ip interface brief | include up
Serial0/0 126.96.36.199 YES manual up up
Now I ping from R2 to R2 no ping I want to know why.???
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 188.8.131.52, timeout is 2 seconds:
Success rate is 0 percent (0/5)
Jon is correct. When you ping you own IP address on a point-to-point serial interface the ICMP packet goes over the wire to the other side and back. So you need to have an IP address on the same subnet on the other router (r3) in order to ping your own interface (r2).
When you ping a serial interface the ping is actually sent down the P2P link to the remote router and back again. Because you have not set an IP address on R3 when R3 receives the packet it does not know how to send it back because it's serial interface is not in the same subnet as the serial interface on R2.
So you need to assign an IP to R3 serial interface in the same subnet as the IP address on the serial interface on R2.