Vista Tutorial - Game via XP-Vista network

Succeeded in getting an XP-Vista network working. Want to play a bridge game
ia one XP and one Vista computer in a network. (works fine between 2 XP
computers. It does not work, whether I use the XP computer as server or the
Vista compute as server. Strange is that if I try to use the XP as the
server, I have to give in the right name (or ip number) and the right port
for the client before I push the connect button. (the xp tells you that he is
waiting for the client) if I give a wrong name, he informs you.
If I do the same on the Vista computer, I can give any name and the computer
tells me after pushing the connect button, that he is waiting.
Who can help me?