I may be wrong here, but I think most large-scale multiplayer games divide everything into lobbies, limited to the maximum number of players that will appear in a given session. Therefore you may have to run multiple instances of your hosting program if you are the host server, which would actually only require packet sending, unless you yourself are playing in one of the lobbies, which would only require one instance of the game along with all the the other packet distribution duties. Yikes! That was a mouthful.
I may be way off here. I have only programmed for multiplayer before (a sweet little tank game) but I have never launched it as such.