The game uses UDP hole-punching to get past router firewalls which works 90%+ of the time, but some routers are freaks that don't allow it (Symmetric NATs). If you want to set up port-forwarding see below.
The default port used by servers is 5728/udp. The client will try to use the first available UDP port from 6000 to 7000. Both the client and the server will use port 80/TCP (http) for, for example, looking up the server list.
Be sure to allow
vtserver.exeare permitted to get passed the Windows firewall!
Forward 5728/udp for the server 6000-7000/udp for the client