This report briefly summarises the server-discovery protocol used by Quake III Arena, Wolfenstein Enemy Territory and Quake 4. When requested by the player a game client issues a query to a well-known ‘master server’ which replies with a list of registered (and nominally active) game servers. The client then probes each game server in sequence presenting the player with information about each game server as replies come back. Game servers participate in this process by registering with the master server when they start up. The server discovery process is similar for all three games based on short UDP packet exchanges. A discussion of UDP traffic between game servers and clients during connection and actual game-play is beyond the scope of this report.