One of the five IRC ports in IANA's 6665-6669 ircu range — an alternate to 6667.
IANA assigns 6665-6669/tcp as a single block named ircu, after the Undernet IRC daemon, and RFC 7194 confirms plaintext IRC is covered by that assignment. Most networks listen on all five so clients have a fallback when one is blocked, and 6666 is the most commonly offered alternate. Its memorable digits also made it a long-standing favourite for command-and-control channels of IRC-controlled bots, which is why some corporate filters block the whole range outright.
Expose with care
Plaintext IRC — nick, credentials sent to services, and every message travel unencrypted. Prefer 6697 with TLS (RFC 7194) for anything you care about.
$ printf 'QUIT\r\n' | nc -v TARGET 6666Seen 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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