No IANA registration and no common service — an open 5811 is whatever local application claimed it.
5811 sits in a gap of the IANA registry: 5813 (icmpd) and 5814 (spt-automation) are registered, but 5811 is not, and no mainstream product defaults to it. Under VNC's 5800+display convention it would correspond to display :11, which is rare in practice. Treat a hit here as an unknown service and identify it directly.
Expose with care
An unknown listener deserves no default trust. Confirm the process and its authentication before exposing it anywhere.
$ sudo lsof -nP -iTCP:5811 -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.