If you want to emulate multiple seperate clients originating on the same system, you can virtualize it by using a program such as VMWare or virtualPC, so you can run multiple simultaneous (sandboxed) operating systems at the same time.
At an application level, they will appear as entirely seperate machines, so great for emulating networks as well.
At an application level, they will appear as entirely seperate machines, so great for emulating networks as well.