Registered to a long-discontinued IDS product; in practice a free high port applications squat on.
IANA assigns 9988/tcp the service name nsesrvr, described as the "Software Essentials Secure HTTP server" — a component of the SecureNet Pro intrusion-detection product, which has not been a going concern for many years. The matching UDP entry is simply Reserved. Nmap sees 9988/tcp open on roughly 0.015% of hosts, and vanishingly few of those are SecureNet. Read an open 9988 as an unidentified application, not as the registered service.
Expose with care
The registration tells you nothing about what is actually listening, so it tells you nothing about whether it authenticates. Fingerprint before exposing.
$ nmap -sV -p 9988 TARGETSeen 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.