How to create an IP Alias in Mac OS X using ifconfig
In some trickier network situations, you may be assigned an IP but need an entirely different IP address to access certain network resources, but how on earth do you have two IP addresses at once? Create an IP Alias! It’s easy to do with the Mac OS X command line utility ifconfig.
Type the following at the command line prompt to alias the new IP of 192.168.0.101 to your existing subnet mask
sudo ifconfig en0 alias 192.168.0.101 255.255.255.0
If you want to get rid of the alias, just type:
sudo ifconfig en0 -alias 192.168.0.101