It's probably your NAT router, you need to forward the D3 ports to your machine to make hosting servers work.
Instructions for port forwarding on most routers (and the basic concept of what port forwarding is) can be found here:
The relevant port for Descent 3 is usually: 2092 (but can be changed).
The port needed for your server to show up on a relevant tracker is usually: 27900 (but can also be changed).
Use the D3 Server tool to change these ports if you want too, since I don't recommend doing it manually these days.
(19:11) [D3k]Gooberman: pffft, I didnt get owned baal, you just got 60 lucky fusion shots