things that have caused me grief when trying to host MP online

(just listing everything that has ever blocked me from hosting)
(kinda like thinking out loud)

an anti-spyware package could be blocking the game from using ports

an anti-virus package could be blocking the game

i had a spare PC acting as firewall/router, i could find no way to port forward through it
(that was a software issue with the program i was using to emulate a router)

some routers need to be specifically configured to allow port forwarding
(some modern cable/adsl modems are also routers, usually they dont need
configuring when used as modems, but maybe they think they are a router)

one ISP i was with, their Proxy, i had to speficially name / address in internet explorer

another ISP Proxy i had to cut out of the "chain" as it simply would not allow the game to connect



there has to be a reason for your problem, if we can find it I will be very happy,
the more info we have on glitches like this, the better we can support people who want to play

cheers
B.