What IP address are we talking about here?

If you have several machines connected to a router, you can fool the router into thinking that something came from a different (legitimate) machine.....I guess those are internal addresses as they are (at least mine are).

From your ISP I don't think just uses the router's IP address..........mess with that and you won't get a reply, so no connection? As far as I am aware if I am at 935 Pennsylvania Avenue and have an IP addy allocated to that, and I spoof a message from the IP for 1600...........I won't get the reply, but Mr Obama will?

NOTE: I am just going by how my stuff seems to work over here in the UK.

I have no idea how a VM is handled..............I guess my stuff would just look at the machine, and not worry about the OS.........after all, a router just does what it directs traffic. I guess the question is whether the VM just allows a different OS on the same hardware, or whether it presents itself as a new device to the outside world?

I do not run VM, so I don't know for sure. The answer might also be different for Cat5/RJ45 connections as opposed to wireless? I believe that 2 of my wireless connectors can also act a sub-routers???

It might help if you could suggest some sort of hypothetical scenario?

Please don't be surprised if you don't get a reply other than this.........your spoofing question really is a completely different issue, that merits a new thread.