Registered to the X.Org print server, a subsystem that no longer ships.
IANA assigns 8100 on TCP and UDP to xprint-server, the Xprint printing extension that shipped with XFree86 and Solaris and was removed from X.Org years ago. You will not meet a live Xprint server today. What you find on 8100 now is a locally chosen listener — it is a round number in the 80xx band, so applications and containers take it as an alternate HTTP or service port.
Expose with care
The registration is dead history; identify the actual service before deciding anything about it.
$ nc -vz TARGET 8100Seen 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.