Port 251 has no assigned service. It belongs to the well-known ports range (0-1023) but has never been claimed by any protocol. No RFC defines what should run here. No software listens here by default.
What the Well-Known Range Means
Ports 0-1023 are called "well-known" or "system" ports. They're assigned by the Internet Assigned Numbers Authority (IANA) through formal procedures—typically requiring IETF review or IESG approval.1 These ports are meant for standard Internet services: HTTP on 80, HTTPS on 443, SSH on 22, SMTP on 25.
But not every port in this range has been assigned. Port 251 is one of the gaps.
Why Keep It Reserved?
IANA reserves unassigned well-known ports for potential future standardization. The theory is that someday, someone might design a protocol important enough to deserve a well-known port number, and when that happens, port 251 (and others like it) will be available.
In practice, most of these empty ports will probably stay empty. The Internet's core protocols were defined decades ago. New services rarely get well-known ports anymore—they use registered ports (1024-49151) or dynamic ports (49152-65535) instead.
Should You Use This Port?
No. RFC 6335 is clear: "Users MUST NOT deploy implementations that use assigned port numbers prior to their assignment by IANA."2 Even though port 251 is currently unassigned, using it for your own service is a bad idea. If IANA ever assigns it to a standard protocol, your custom application would conflict with legitimate traffic.
If you need a port for internal or private use, use the dynamic/private range (49152-65535). Those ports are explicitly designated for temporary or private use.
How to Check What's Listening
If you want to see if anything is actually listening on port 251 on your system:
On Linux or macOS:
On Windows:
Most likely, you'll find nothing. This port is almost certainly closed on your system.
The Silence of Empty Ports
There are currently dozens of unassigned ports in the well-known range. They exist as placeholders—addresses reserved for protocols that don't exist yet and may never exist. Port 251 is one of these silent spaces: a number in a registry, a socket that never opens, a door that leads nowhere.
The Internet's addressing system has room for 65,535 ports per protocol (TCP and UDP). Not all of them need to be used. Some are just gaps in the numbering—reserved but unclaimed, available but unwanted.
Port 251 has been empty since IANA started keeping records. It will probably stay that way.
Related Ports
Other unassigned well-known ports include many scattered throughout the 0-1023 range. Each one sits waiting for a protocol that might never come.
The assigned ports nearby:
- Port 250: Previously unassigned (like 251)
- Port 252: Previously unassigned
- Port 256: Previously unassigned
Most of the well-known ports in the 200-300 range remain unassigned. The early Internet protocols claimed the low numbers (0-100), and the high numbers in this range were never needed.
Frequently Asked Questions
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