The game gives your internal router adress if you use a router,...which will not work for playing MP. You will need to visit
www.whatismyip.com to see what the actual IP you need to use to connect with.
Firewalls have to be disabled or the game allowed to be an exception. Port 1941 needs to be open if you use a router. So far nobody that has claimed they had port 1941 opened has been able to host unless they DMZ'ed their machine on their router settings page. I am not sure if this is because they dont know how to open port 1941 correctly or becasue something other than port 1941 needs opened also. I always DMZ my machine to play oline games, so I never learned how to open individual ports.