Well NAT hole punching for UDP works on ~90% of the routers, but may fail on routers that use port-restricted NAT; AFAIK in that case at least one player must set up port forwarding on the router otherwise the NAT tables on the router will discard the DC traffic.
You should either ask Larian about the port numbers or run "netstat -a -b -n" while playing with other players connected and set up port forwarding for all entries where the state is "LISTENING" and has the DC exe filename under it.
