In the gaming sector, master servers are used to manage internet servers. In general there are mechanisms implemented, that allow a user to put up a game server and make this game server known to the public by reporting it to one or more master servers. These servers keep a record of currently known game servers. When this list is requested by a client, the server sends it to the client. Often the list of servers is filtered in advance, for compatible protocols/versions, currently played game modes, rules or whether the server is full, empty or uses bots. Depending on the game, the available master server addresses may be hard-coded (cannot be changed easily) or configured (can be changed in some sort of configuration).
Master servers should not be confused with the actual game servers, which host the matches. A variation of the master server concept are match making servers.
Depending on the game, the access to the master server can be free, e.g. for Star Trek: Voyager Elite Force. Some games, however impose a kind of access control of the clients to the server. E.g. Unreal Tournament 2004 requires the game to prove it is run with a not black-listed CD key. If the client fails to pass this test, no servers are sent back to the client.
As games heavily benefit from this concept (no manual entering of already known IP addresses, no bookmarking required), for some games the loss of »their« master server (e.g. the publisher decides to shut them down permanently) means the end of convenient multiplayer (or the end of multiplayer altogether). This makes clear, that having some sort of backup or fall back option may be the last means of keeping games alive. For some games there are replacement servers available. One example is https://333networks.com/. Currently (2022-02) they support 14 different games.