Unregistered and rare — Nmap finds it open on roughly two hosts in ten thousand.
There is no IANA registration for 40911 and no service name in Nmap's database, which measures its open frequency at about 0.02%, near the bottom of the ports Nmap bothers to scan by default. Nothing conventional runs here, so an open port is either a bespoke service or an ephemeral socket in the Linux 32768-60999 range. The number itself tells you nothing.
Expose with care
Rare and unidentified is not the same as harmless; fingerprint the listener before making any exposure decision.
$ nc -vz -w 3 TARGET 40911Seen 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.