Registered to the Display Support Protocol, a service you will effectively never meet.
IANA registers port 33 as 'dsp — Display Support Protocol', assigned to Ed Cain, on both TCP and UDP. There is no RFC behind the registration and no reference implementation in common use. Nothing on a modern system binds it, so a listener here is almost certainly unrelated software that picked a free low port. Treat the registration as a name in a table, not as a prediction of what you found.
Expose with care
The registered protocol is effectively extinct, so whatever answers on 33 is unidentified and should be named before it is exposed.
$ nc -vz TARGET 33Seen 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.