IANA-reserved, and the last port of the privileged 0–1023 range that only root may bind on Unix.
RFC 6335 defines 0–1023 as the System (well-known) ports, which on Unix-like systems only a privileged process may bind, and IANA holds 1023 itself Reserved rather than assigning it. So no standard service lives here. Where 1023 does show up is as a source port: rsh, rlogin and NFS-era clients deliberately chose a reserved source port in 512–1023 as proof the caller was root on a trusted host. A listener on 1023 is a deliberate choice by something running with privilege.
Expose with care
No assigned service means nothing to allow by default. The privileged-source-port convention it belongs to was host-based trust, which is not a security control on a modern network.
$ sudo lsof -nP -iTCP:1023Seen 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.