What Port 2931 Is
Port 2931 sits in the registered port range (1024–49151). These ports are tracked by IANA, the Internet Assigned Numbers Authority, which maintains the official list of which port belongs to which service. Unlike the well-known ports below 1024 — where HTTP lives at 80, HTTPS at 443, SSH at 22 — registered ports don't require root privileges to bind, and their assignments carry no enforcement weight. Anyone can request one.
Someone did. Port 2931 is registered to Circle-X, on both TCP and UDP.
The Ghost Registration
The IANA entry for port 2931 lists:
- Service name: circle-x
- Transport: TCP, UDP
- Contact: Norm Freedman, normfree@worldnet.att.net
AT&T WorldNet was an Internet service provider that shut down around 2009. The email address no longer exists. The service it points to — Circle-X — has no RFC, no public documentation, no source code, no archived website. It was registered, presumably for a real purpose, and then vanished.
This is not unusual. The registered port range contains hundreds of entries like this: names without services, services without users, projects that never shipped or quietly died. The IANA registry is partly a historical archive of ambition.
What Actually Uses Port 2931 Today
Monitoring from the SANS Internet Storm Center shows minimal scanning activity on this port — a handful of probes per day, typical background noise. No known malware families use it. No common server software binds to it by default.1
In practice, port 2931 is likely sitting empty on almost every machine on the Internet.
How to Check What's Listening on This Port
If you want to see whether anything on your own system is using port 2931:
macOS / Linux:
or
Windows:
If nothing comes back, nothing is listening. That's the expected result.
Why Unassigned (and Forgotten) Ports Matter
The registered port range exists to prevent collisions — if two applications both want port 5432, one of them has to move (PostgreSQL got there first). But registration is first-come, first-served with no ongoing requirements, no proof of use, no expiration. A port can be registered for a project that never launched and stay registered forever.
This creates a subtle pressure in network design: when you're picking a port for a new service, you have to navigate around thousands of entries that may or may not correspond to anything real. The registry is the map, but the territory and the map diverged a long time ago.
Port 2931 is safe to use in internal networks and private applications. Just know that if Circle-X ever comes back from wherever it went, there could technically be a conflict.2
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