My guess is that you have one broadband internet connection that you share through a wireless router. You connect via a wire and your brother through wireless. You have the same external IP address but different internal IP addreses. Your router uses NAT so you both can play the game. To the external world you are both on the same IP address, the external one. I hope this clears up the confusion. Interestingly, I guess you can easily trade between yourself and your brother (even offline) which is what the online gamers don't want.