Unassigned at IANA — no registered service, no common squatter.
IANA lists port 6 as Unassigned on both TCP and UDP. It has no assignee, no reference document, and no daemon that binds it as a default. The number is easy to confuse with IP protocol number 6, which is TCP itself, but that is a different registry entirely. A listener here is bespoke, and identifying the owning process is the only reliable step.
Expose with care
Unregistered, so an open port 6 tells you nothing on its own; name the process before judging exposure.
$ sudo lsof -nP -iTCP:6 -sTCP:LISTENSeen 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.