Registered as mecomm, with no description and nothing known to implement it.
IANA lists TCP and UDP 668 under the service name "mecomm" with only "MeComm" as the description, no reference document, and no assignee note; the same bare entry ships in the /etc/services file on macOS. No maintained software is known to use it. The honest answer for this port is that the registration is all there is, and any open 668 needs to be identified from its banner or process, not from the port number.
Do not expose
An unexplained listener on an otherwise unused system port — find out what it is before it faces anything.
$ nc -vz -w 3 TARGET 668Seen 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.