The default HTTPS listener for GlassFish and Payara, and IANA's Intermapper port.
Eclipse GlassFish ships an HTTPS listener on 8181 next to its HTTP listener on 8080 and the admin console on 4848; Payara inherits the same defaults. IANA registers 8181/tcp to intermapper, HelpSystems' network management system. In practice 8181 is also a common second HTTP port for OSGi containers and Java web stacks generally, so a listener here may speak TLS or plain HTTP depending on what put it there. Probe it both ways before concluding.
Expose with care
If it is GlassFish, the interesting target is the admin console on 4848 next door; a default-certificate 8181 also gives you no identity assurance.
$ openssl s_client -connect TARGET:8181 </dev/null 2>/dev/null | openssl x509 -noout -subject -datesSeen 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.