The default source IP address when telnetting from a Cisco router is that of the interface closest to the destination. Because some access lists may block traffic from the default IP address, it may be useful to temporarily or permanently set the telnet source IP address. This also helps test access lists.
To temporarily set the telnet source IP address for a single telnet session, use the /source-interface extension to the telnet command.
For example, to telnet to 192.168.1.4 using the source IP address of the interface FastEthernet0:
telnet 192.168.1.4 /source-interface FastEthernet0
To set this interface as the source IP address for all telnet sesions, use this command in configuration mode (conf t):
ip source-interface fastethernet0