Not registered with IANA and not tied to any known protocol.
Port 720 has no entry in the IANA service name and port number registry, and Nmap's service database lists it as "unknown" with a very low observed frequency. There is no well-documented application, malware family, or convention that claims it. Anything listening here is a locally chosen port — an internal service, a test harness, or a tunnel endpoint. The only way to know what it is, is to fingerprint it.
Expose with care
An unregistered port tells you nothing about what is behind it; identify the service before deciding whether it belongs on a public interface.
$ nmap -sV -p 720 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.