Network Changes in GameRangerGameRanger has introduced a significant new feature that bypasses the common problems that modems and routers cause with hosting (and sometimes joining) games. No setup or configuration is required, and all just works automatically.
In rare cases, some users with certain types of routers may still have problems joining or hosting, particularly if they have firewalls that are explicitly blocking ports. You will know if you are having network issues before you even launch the game because your name will remain in italics in the game room even after 10 seconds after joining the game room. If you can join other rooms or host games for other people just fine, it may be that host that has the network problems.
If you do have a troublesome router, you can fix the problem by configuring the router to enable Universal Plug and Play (UPnP), or manually do port forwarding for UDP port 16000. See Hosting Help for information about port forwarding. If you have a firewall, you'll need to stop it from blocking that port.