Unassigned at IANA, but seen open more often than its neighbours in Nmap's scan data.
IANA lists the block 6641-6652 as Unassigned, so 6646 has no registered service. Nmap's service database gives it no name either, yet records it with an open-frequency an order of magnitude above the surrounding unnamed ports — meaning real software binds it in the field without any registration behind it. Endpoint-security agents are the usual suspects on Windows hosts, but no vendor documentation pins the port down, so identify the owning process rather than guessing from a name table.
Expose with care
An unregistered port that is nevertheless commonly open is exactly the kind of listener worth naming before you leave it reachable.
$ sudo lsof -nP -iTCP:6646 -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.