IANA calls it cbt, but everyone else knows 7777 as the default game-server port — Unreal Engine dedicated servers start here.
Epic's documentation states that an Unreal Engine dedicated server "listens on the localhost IP address (127.0.0.1) at port 7777" by default, and the convention spread across the games built on it and its contemporaries, which is why 7777 is the number people reach for when standing up any self-hosted multiplayer server. Game traffic is normally UDP; a TCP listener on 7777 is more often an application server, an RCON or query interface, or something unrelated that liked the digits. The IANA registration (cbt, Core Based Trees multicast routing) has no bearing on any of this.
Expose with care
A public game server is exposed on purpose, but it is unauthenticated by design and mod or plugin layers are a repeated source of remote code execution — run it isolated, patched, and never on a machine that hosts anything else.
$ nmap -Pn -sU -sV -p 7777 TARGETSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.