An IANA registration with no public protocol behind it.
IANA registers 2251 as dif-port, "Distributed Framework Port". No specification was published with the registration and no commonly deployed software defaults here, so the entry is a reserved name rather than a protocol you will meet on a scan. A listener on 2251 is best identified by process, not by the registry.
Expose with care
Unknown service: find out what bound the port and whether it authenticates before exposing anything.
$ sudo lsof -nP -iTCP:2251 -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.