The control channel of Netscape's Java Transaction Protocol — a dead product's registration.
IANA registers 1687 as "nsjtp-ctrl" (TCP and UDP), the control half of Netscape's JTP, paired with "nsjtp-data" on 1688. It belonged to Netscape's application-server line, which passed through iPlanet and Sun and no longer exists. There is no current implementation and no public specification, so the registration is historical only.
Expose with care
Nothing in current use binds 1687, so an open port here is an unidentified listener that needs to be traced to a process before you judge it.
$ nc -vz -w 3 TARGET 1687Seen 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.