Unassigned, unnamed, but scanned often enough to rank high in Nmap's frequency table.
IANA places 20828 in the unassigned block 20811-20998. Nmap lists it as 'unknown' yet with one of the higher open-frequencies among unnamed ports in this range, meaning internet-wide scans found it open on a measurable number of hosts without ever identifying what answered. No public protocol, product default, or RFC claims it. Treat an open 20828 as an unidentified application until a version scan says otherwise.
Expose with care
Unnamed but commonly-open ports attract opportunistic probing. Fingerprint the listener and apply that service's rules.
$ nmap -Pn -sV -p 20828 --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.