Unassigned by IANA; anything listening is a local or vendor-specific choice.
IANA has no registration for 306, and no public protocol claims it. Nmap lists it as "unknown" while still including it in wide scans because a measurable share of hosts answer, which usually points at embedded devices or a niche product rather than anything standardised. Do not read a service into the number — there is not one to read. The only honest answer is the registration status plus a service probe.
Expose with care
No registered protocol means no assumption of authentication or encryption; probe it and treat it as untrusted until identified.
$ nmap -sV -p 306 TARGETSeen 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.