First i must ask what kind of router are you behind and if any other software or hardware firewall, though some routers have a built in firewall. You might have to talk to your ISP about all the details of hosting. Also if you have purchased an IP there would inlie your problem since you changed the IP of the server machine. Definately recommend checking hardware/software specs and talking to your ISP. Good way to double check the Internethosting try hosting a LAN game if you can, this will narrow down where the problem lies