Status: Reserved (removed from assignment May 18, 2017)1
Range: System Ports (0-1023)
Protocols: TCP and UDP
Service Name: None
What Reserved Means
Port 252 sits in an unusual state. It's not unassigned—you can't request it for a new service. It's not assigned—no protocol currently uses it. It's reserved, which means IANA has marked it as unavailable for regular assignment.2
Most ports in the well-known range (0-1023) are either actively assigned to protocols or simply unassigned and available. Port 252 is neither. The IANA registry shows it was explicitly removed from assignment on May 18, 2017.
The System Ports Range
Port 252 falls within the system ports range (0-1023), historically called "well-known ports." These are assigned through IETF Review or IESG Approval—formal processes requiring standards documentation.2
This range contains the Internet's foundational protocols: SSH on 22, HTTP on 80, HTTPS on 443. Port 252 exists among these giants, but carries nothing.
What We Don't Know
The registry doesn't say what port 252 was assigned to before 2017. It doesn't say why it was removed. Reserved ports are typically held back for special purposes—extending port ranges, future standards work, or retiring problematic assignments.
Someone made a decision to remove port 252 from active use and reserve it. That decision is recorded in the registry as a date: 2017-05-18. The reason isn't.
Why Reserved Ports Matter
Reserved ports serve as a buffer in the port system. They prevent accidental reassignment of ports that were problematic, prevent conflicts with legacy systems, and hold space for future standards work that might need specific port numbers.
Port 252's reservation suggests it has history—something once ran here. The reservation prevents that history from repeating.
Checking What's Listening
Even though port 252 is reserved, you might find something listening on it on your system. Unassigned and reserved ports can still be used by local applications.
On Linux or macOS:
On Windows:
If something is listening, it's either a local application that chose an unused port, or legacy software from before the 2017 removal.
The Architecture of Absence
The Internet's port system contains three types of ports: those actively carrying traffic, those waiting to be assigned, and those deliberately held back. Port 252 is the third kind—present in the registry not because it's used, but because it was removed.
Most unassigned ports were never claimed. Port 252 is different. It was claimed, used for something, and then retired. The reservation is what remains—a placeholder marking where something used to be.
We don't know what it carried. We only know someone decided it should stop.
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