Unassigned by IANA; /etc/services still lists it as newacct and flags it as unauthorized use.
IANA marks TCP 100 as Unassigned. Most Unix /etc/services files nonetheless carry the line newacct 100/tcp # [unauthorized use] — a squatted "new account" service that was never registered, and the comment is IANA's own note that the usage is unsanctioned. So a scanner may label an open 100 "newacct" while no such protocol is defined. Treat the label as historical trivia and fingerprint whatever is actually listening.
Expose with care
There is no standard service to reason about here; an open 100 is an unknown listener until you identify it.
$ nc -vz TARGET 100Seen 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.