No IANA registration and no established service; a round number that ad hoc software likes.
5850 has no entry in the IANA service registry and no widely deployed default. Round numbers like this get chosen by hand for internal services, staging deployments, and one-off proxies, which is the usual explanation for finding one open. Nothing about the port number itself tells you the protocol, so probe the service or check the process.
Expose with care
An unidentified listener, most likely something internal that was never designed to face the internet. Confirm before exposing.
$ sudo lsof -nP -iTCP:5850 -sTCP:LISTENSeen 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.