Reserved at IANA on TCP; remembered as the Napster and WinMX file-sharing port.
IANA marks 6699/tcp as Reserved with no assignee, while 6699/udp was registered in 2020 for the Babel routing protocol over DTLS (RFC 8968). The TCP side is famous for something else entirely: nmap's service table still names it napster, from the era when Napster-style and WinMX clients defaulted to this port for peer connections. Those networks are gone, so an open 6699/tcp today is either a long-abandoned client or unrelated software on a free port.
Expose with care
A live listener here is worth investigating — either an unmaintained file-sharing client or something you have not identified. It is not a service you should be deliberately publishing.
$ sudo lsof -nP -iTCP:6699 -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.