Unregistered; the only true statement about a listener here is that something picked the number.
IANA does not assign 18101 — the closest registration in the range is 18104 (radpdf, RAD PDF Service), which is a different port. Nmap has no service name for 18101 and scans it by default only because its measured open-port frequency puts it in the top 1000. No widely deployed software documents it as a default.
Expose with care
No known protocol means no known authentication or encryption story. Fingerprint before you decide, and default to closed at the perimeter.
$ nmap -Pn -sV -p 18101 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.