Internet-Security.com TCP port 666 details

Doom (Id Software): IANA assigns TCP/UDP port 666 to “doom”; while the original game mostly used IPX, some TCP/IP adapters and early servers honored this assignment for multiplayer.
airserv-ng (Aircrack‑ng suite): by default listens on TCP 666 to expose a wireless interface to remote tools like airodump‑ng and aireplay‑ng.
Security note: Port 666 has also been a popular choice for backdoors/botnets to hide listeners, so an unexpected service on this port can indicate compromise.

TCP 666