If you are using a FireWall, a NAT (sometime your provider use this for cable connection) or AOL it's possible that other player can't see you, because request are rejected by your PC.
- For a firewall, disable it during play, or if you know how, authorize connections to Quake2 port (27910)
- For NAT, there is nothin to do, you can only play alone or join other players
- For AOL ... change for a "real" provider