As a fun aside, you can do this on localhost by defining additional addresses tied to the loopback interface:

    127.0.0.2
    127.0.0.3
    etc.

I once had to test a piece of software that identified its connections by the source IP, so I had a script to create thousands of loopback addresses within 127.0.0.0/8 and then run against that software and verify the connections were doing what they were supposed to do. (This was on Mac OS X.)