CANNOT HOST NETWORK GAMES
1. ENSURING THE NEEDED PORTS ARE OPENED
Every effort has been made in the games to open network ports to allow people to connect to your created game.
Sometimes however this will have to be done manually.
Sometimes, for reasons unknown at this time, people will just not be able to connect to you.
See this forum thread and follow all instructions in it to try to open ports.
2. DISABLING DHCP
If all else fails, try disabling DHCP in your internet connection setup.