Registered twice — as mdqs, and to id Software for Doom — but neither is what you will find running.
IANA carries two registrations for 666 on both TCP and UDP: "mdqs", with no description, and "doom", assigned to id Software. The Doom entry is a 1990s artefact from the era when games registered ports; Doom's own multiplayer ran over IPX, and modern source ports such as Chocolate Doom and GZDoom use their own UDP ports rather than 666. Because the number is memorable and unclaimed in practice, it gets picked deliberately by hobby projects and by scanner and malware authors alike, so treat a listener here as unidentified until you fingerprint it.
Do not expose
Nothing standard listens here, so an open 666 is an unknown service you have not accounted for — identify it before deciding anything else about it.
$ nc -vz -w 3 TARGET 666Seen 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.