multplayer

I am trying to use multiplayer. I have fshost and when I pick a server it says to ensure that certain ports are added. What is that all about? And how do I add these ports. I noticed that the ip address is the same for all games within a server but the ports are different. Does that mean that you have to add the ports that are required for each game and when finished with that game remove those ports? I would appreciate some help with this. I am using windows 7 and fs2004Ralph33