Unassigned at IANA; a round-number spare HTTP port people pick by hand.
IANA lists 8098 and 8099 together as Unassigned, so nothing is registered here. 8099 gets chosen the way 8080 and 8888 do — it is memorable and usually free — which makes it a common manual pick for a second web app, an internal tool, or a test server. There is no protocol implied by the number.
Expose with care
Hand-picked ports are usually hand-configured services; confirm it authenticates before treating it as safe to reach.
$ curl -sI --max-time 5 http://TARGET:8099/Seen 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.