Registered since 2017 to a model-railway control protocol; before that, RSA's remote admin daemon.
IANA reassigned 5550/tcp in 2017 to CBUS, the MERG (Model Electronic Railway Group) message protocol for controlling model railway layouts over a network. The earlier occupant, still present in old services files and firewall rules, is sdadmind, RSA ACE/Server's remote administration daemon. UDP 5550 is reserved. Which one you are looking at depends entirely on the host: a hobby layout controller or a legacy SecurID server.
Do not expose
Both plausible occupants are control planes — layout control or authentication-server administration — and neither should be reachable from the internet.
$ nc -vz -w 3 TARGET 5550Seen 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.