No IANA registration and no documented default — an unidentified high port that scanners probe by habit.
18040 is unassigned in the IANA registry and unnamed in Nmap's services database, but it is inside Nmap's default top-1000 TCP set because of observed open-port frequency. The registered ports nearby (18000 biimenu, 18104 radpdf) have no relationship to it. Whatever listens on 18040 chose the number itself.
Expose with care
Identify the process first. An unknown listener reachable from the internet is worth closing until someone can name it.
$ sudo lsof -nP -iTCP:18040 -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.