wivern: Yes... what I was trying to do was clarify that without making it sound too difficult (tricky when dealing with TCP/IP)
Looking back at what I wrote, I think I wrote too much. Anyway, I just didn't want people getting the idea that they could effectively "hide" themselves with a masked or cloned MAC. Without fully understanding what's going on you could really screw things up. I wish NIC vendors would stop calling their MAC addresses configurable though. It's a little misleading. And I don't know how this thread branched off on to TCP/IP. Suzy83 is probably going to get a headache reading all of this.
If they're still reading the thread.
broku: If MSN treats peer-to-peer connections like a regular network does then YES you can get an IP by sending them a file. I'm not sure how MSN works but for the sake of argument if MSN works like a router or switch for messenger users then your IP packets must contain a destination MAC and IP. Therefor, when your computer handshakes with their computer to establish a session you will have their IP. But like I said... I don't know how MSN works so you could be right.