A Directory server is provides a way for clients to retreive a list of zones that are currently online and running. These servers provide information on any zones that have registered with the Directory server. This information includes the name and description of the zone, the ip and port that should be used to connect to the zone, the server version, whether or not a billing server is connected to the zone, as well as the number of players in the zone.
Zones can register with Directory servers so that players can find the zone. This is done on UDP port 4991 using the Directory Game Server Protocol.
Directory Server Software
The main Directory Server Software Available
JEDirectoryServer Public Directory Server software v0.3 (by Prittk) http://www.subspacedownloads.com/index.php?act=list&cat=3