Neverwinter Nights requires each player in a local LAN game to own a copy of the game, if any of the clients have external Internet access. This requirement seems a little unreasonable for casual gaming. Luckily, a simple solution exists under Windows to disable server authentication. Simply edit the file:

%windir%\system32\drivers\etc\hosts

and add the following lines to remap the Neverwinter Nights official servers to localhost:

127.0.0.1       nwmaster.bioware.com
127.0.0.1       nwnauth.kr.infogrames.com 

Happy gaming!