Port 331 is unassigned. No service, no protocol, no RFC. It's a number in the registry that no one has claimed.
What This Means
Port 331 falls within the well-known ports range (0-1023), also called system ports. This range is managed by the Internet Assigned Numbers Authority (IANA) and reserved for fundamental Internet services. Ports in this range require special privileges to bind to on Unix-like systems—they're meant for infrastructure, not experiments.
Yet port 331 has no assignment. The IANA registry simply marks ports 325-332 as "Unassigned."1
Why Unassigned Ports Matter
The well-known ports range is finite—only 1024 numbers. Most of them carry critical services: HTTP on 80, HTTPS on 443, SSH on 22, DNS on 53. These are the numbered addresses of the Internet's nervous system.
But not every port needs a service. Port 331 sits empty, and that's fine. The Internet has grown beyond the original assumption that every well-known port would eventually be claimed. Some gaps remain, and they serve a purpose: they're available if needed, but they don't clutter the network with unused infrastructure.
No Known Unofficial Uses
Unlike some unassigned ports that get adopted by applications or unfortunately by malware, port 331 has no documented unofficial uses. No trojans are known to use it. No legitimate software has claimed it as a default.2
It's genuinely quiet.
How to Check What's Listening
If you want to see whether anything is listening on port 331 on your system, use these commands:
On Linux or macOS:
On Windows:
If nothing appears, the port is closed. If something does appear, you've found a service running on an unassigned port—unusual, but not impossible.
The Architecture of Empty Space
The Internet's port system wasn't designed with every number filled. RFC 6335, which defines how IANA manages the port registry, explicitly allows for unassigned ports.3 They exist as potential—space for future protocols, or simply space that will never be needed.
Port 331 is part of that space. It's not waiting for a specific service. It's just available, like a numbered seat in a theater that no one has reserved.
Related Ports
- Port 330: Also unassigned
- Port 332: Also unassigned
- Ports 325-329: Unassigned
- Port 333: Assigned to texar (Texar Security Port)
Frequently Asked Questions
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