The last number in an unassigned IANA block, sitting directly below the registered 20222.
IANA's unassigned range 20203-20221 ends here, and the very next port, 20222, is registered as iPulse-ICS — so 20221 is often scanned as the low half of a presumed pair even though nothing is assigned to it. Nmap carries it as 'unknown' on both TCP and UDP with a small open-frequency, which is enough to put it in the default top-1000 scan. There is no documented protocol for this number.
Expose with care
Nothing standard listens here, so an open port is an unknown application. Fingerprint it and judge that, not the number.
$ nmap -Pn -sV -p 20221 --reason TARGETSeen 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.