Unassigned by IANA; historically the diagnostic telnet port of Network Computing Devices X terminals.
IANA lists 5995-5998 as Unassigned, so there is no current owner. The name that persists in scanner databases is ncd-diag — nmap still labels 5997/5998/5999 as the preferences, diagnostic and configuration telnet ports of Network Computing Devices X terminals, thin clients from the late 1980s and 1990s. That hardware is long gone, so a listener today is an unrelated application that picked a free port just below 6000.
Expose with care
No registered service means the port number tells you nothing — identify the process locally before allowing any access. If it genuinely is a telnet-style diagnostic console, treat it as never.
$ nmap -Pn -sV -p 5998 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.