IANA has never assigned this port, and no common service is known to squat it.
7921 falls inside the block IANA lists as 7914-7931 Unassigned, so there is no registered service name to look up. Nmap's service database has no name for it either and files it as unknown, seen on roughly two hundredths of a percent of scanned hosts. In practice that means anything listening here is some application's private choice of port, not a standard protocol. Identify it by its banner rather than by the number.
Expose with care
An unregistered port tells you nothing about safety — find out what the listener actually is before deciding, and treat an unidentified internet-facing service as untrusted.
$ nmap -sV -Pn -p 7921 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.