The workstation-server port of Unisys MAPPER, now sold as Business Information Server (BIS).
IANA registers 3986 as mapper-ws_ethd, "MAPPER workstation server". MAPPER is a fourth-generation language and application environment that came out of Sperry Univac in the 1970s and survives as Unisys BIS on ClearPath and Windows; this port carries workstation clients into the server. It is worth knowing that port scanners print "mapper-ws_ethd" for 3986 straight from the IANA name, so a scan label alone is not evidence that a Unisys mainframe environment is present.
Do not expose
A line-of-business application server for a mainframe-era environment: front it with a VPN rather than putting the workstation protocol on a public address.
$ nc -vz TARGET 3986Seen 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.