Registered to Cisco as an identification port; rarely seen in the wild.
IANA lists 1999 as tcp-id-port, "cisco identification port", registered by Cisco Systems. There is no public protocol specification for it and it is not a service you deliberately turn on. In practice a listener here is either a Cisco device or, more often, an unrelated application that grabbed a round number just below 2000.
Expose with care
Nothing well documented runs here, so treat an open 1999 as an unidentified listener: fingerprint it before deciding, and do not expose an unknown service to the internet.
$ nc -vz TARGET 1999Seen 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.