Port 231 has no official service. No protocol is assigned to it. No RFC defines what it should do. According to the GRC Port Authority database, port 231 is simply "not assigned."1
It's a number in the registry with nothing behind it.
What "Unassigned" Means
Port 231 falls in the well-known ports range (0-1023), also called system ports. These are the ports managed most carefully by the Internet Assigned Numbers Authority (IANA). Getting a well-known port requires "IETF Review" or "IESG Approval"—the most stringent assignment procedures in the port system.2
But port 231 never got assigned.
Unassigned ports are "currently available for assignment upon request, as per established IANA procedures."2 They're not blocked or reserved for special purposes. They're just open—waiting for someone to need them enough to go through the formal process.
Why Unassigned Ports Exist
The well-known range has hundreds of ports, but not all of them are in use. IANA deliberately maintains unassigned ports as a resource for future needs.3
There are several reasons a port might remain unassigned:
Conservation strategy. Port numbers in the well-known range are conserved. IANA has procedures that allow previously assigned ports to become unassigned through de-assignment or revocation. The Internet's address space is finite, and careful management means keeping some doors closed until they're needed.3
Protocol-specific reservation. When a service is assigned a port for TCP but not UDP, IANA marks the UDP side as "Reserved" rather than leaving it unassigned. This prevents conflicts. Truly unassigned ports like 231 have nothing claimed on either protocol.3
Historical gaps. Early port assignments were sometimes inconsistent—services got entire ranges even when they didn't need them, leaving gaps. Port 231 might simply be one of those gaps, preserved as the system matured.3
What This Port Carries
Nothing. If you see port 231 listening on your system, it's not an official service—it's something unofficial, possibly custom software, or potentially something that shouldn't be there.
How to Check What's Listening
On Linux or macOS:
On Windows:
If something is listening on port 231, it's not following an official protocol. It's either:
- Custom software using an unassigned port (which is fine for private use)
- Malware or unauthorized software (which warrants investigation)
Why Unassigned Ports Matter
Port 231 represents something important: restraint. The well-known ports range was meant to be carefully managed, and keeping ports unassigned is part of that management. Not every number needs a service. Not every door needs to be opened.
The Internet's addressing system works because numbers have meaning. When a port is assigned, it carries weight—an RFC, a protocol, a history. Port 231 has none of that, and that's deliberate.
It's a space held open. A number waiting. A door with nothing behind it—yet.
Related Ports
- Port 0: Reserved by IANA, not available for assignment2
- Port 4: Unassigned (another open slot in the well-known range)2
- Port 1023: Reserved, marks the edge of the well-known range3
Security Considerations
If you find port 231 listening on your system, investigate it. Legitimate services don't use unassigned well-known ports without good reason. Check what process is bound to it and whether it should be there.
Unassigned ports are sometimes chosen by malware precisely because they're unexpected. The absence of an official assignment doesn't make them safe—it makes them unusual, and unusual should make you curious.
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