Registered as "TvBus Messaging" and essentially undocumented in public.
IANA registers 2191 as tvbus, "TvBus Messaging". There is no published specification and no widely deployed product that defaults to it, so unlike its neighbour 2190 it carries no recognisable traffic on modern networks. If you find it open, the registration tells you nothing useful — identify the listening process instead.
Expose with care
An unidentified listener is the finding here; the registered name does not establish what is running or whether it authenticates.
$ sudo lsof -nP -iTCP:2191 -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.