Port 1823 has no officially assigned service. The IANA registry lists it as unassigned — no protocol, no application, no RFC. It is an empty slot.
That is worth understanding, not dismissing.
The Registered Range
Port 1823 sits in the registered port range: 1024 to 49151.
This range exists between two more famous neighborhoods. Below it, ports 0 through 1023 are the well-known ports — HTTP at 80, HTTPS at 443, SSH at 22. These are assigned to foundational protocols that predate the modern web. Above the registered range, ports 49152 through 65535 are the ephemeral ports — the temporary ports your operating system assigns automatically when your browser opens a connection.
The registered range is different. It is where applications go to stake a claim. A developer building a new service can formally register a port with IANA, ensuring their application has a consistent, documented home. FTP uses 21. MySQL uses 3306. PostgreSQL uses 5432. These are registered ports with assigned services.
Port 1823 is registered in name only — claimed by the range, occupied by nothing official.
Known Unofficial Uses
No widely deployed application is known to use port 1823 as its primary port. Security databases note it occasionally appears in network traffic without a clear origin, which typically means one of three things: a custom internal application chose it arbitrarily, an ephemeral connection was logged mid-handshake with a misleading port label, or something misbehaving is using it to avoid scrutiny.
There is no credible association with a specific trojan or malware family, despite port 1823 appearing in some broad security watchlists. Those lists tend to be expansive — treat any port you cannot explain as worth investigating, not as evidence of compromise.
How to Check What Is Listening
If you see port 1823 active on a machine, these commands will tell you what is using it.
macOS / Linux:
Linux (alternative):
Windows:
The output will show the process ID. Cross-reference that with your process list to identify the application. If nothing recognizable owns it, that is worth investigating further.
Why Unassigned Ports Matter
The port numbering system only works because most of it is empty.
If every port had an assigned service competing for the same numbers, building new applications would require negotiating conflicts constantly. The unassigned registered ports are the breathing room — available for custom applications, internal services, and future protocols that have not been invented yet.
Port 1823 is not broken or forgotten. It is available. If you are building a service and need a port, this is one of tens of thousands that could be yours.
The emptiness is part of the design.
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