Unreal Tournament verifies CD keys with the keyserver at these two addresses:

   ut2004master1.epicgames.com:28902
   ut2004master2.epicgames.com:28902

(actually I think one is the keyserver and the other is the game list server)

If you have a [k] CD key that is banned at their keyserver, you can still play LAN games where you are connected to the internet by telling your router or firewall software to block access to port 28902.  This will also prevent you from being able to fetch a list of online (internet) games, but that's to be expected.  You'll probably still need unique CD keys for each player on the LAN though.