Unassigned by IANA, immediately above the registered BoKS block.
The registry shows 6504 as unassigned. Its only context is the neighbourhood: 6500-6503 carry the BoKS access-control daemons, and a BoKS domain configured with a base port a few numbers higher would land traffic here. Nothing is registered or documented as owning 6504 itself, so a listener is either a shifted BoKS block or unrelated software that grabbed a free port.
Expose with care
No registration to reason from — identify the process first. If it turns out to be a BoKS daemon, treat it as a control plane and keep it internal.
$ sudo lsof -nP -iTCP:6504 -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.