Port 255 is reserved by the Internet Assigned Numbers Authority (IANA) and currently has no assigned service. It sits in the well-known port range (0-1023) but remains deliberately unassigned—a blank space held for future possibility.
The Well-Known Range
Ports 0-1023 are called well-known ports, assigned by IANA through formal IETF review processes. These are the ports that carry the Internet's core protocols: HTTP on 80, HTTPS on 443, SSH on 22, DNS on 53. Getting a well-known port number is difficult and reserved for protocols that will be widely deployed across the Internet.
Port 255 exists in this prestigious range but carries nothing. It's marked "Reserved"—a designation that means IANA is holding it for special purposes, not available for regular assignment.1
Historical Context: Experimental Functions
In the early days of TCP/IP, port numbers had explicit range meanings. RFC 758, one of the first documents to describe TCP port numbers, divided the space into four categories:
- Ports 0-63: Network Wide Standard Functions
- Ports 64-127: Host Specific Functions
- Ports 128-223: Reserved for Future Use
- Ports 224-255: Any Experimental Function
Port 255 was originally in that experimental range—a space where new protocols could be tested without formal assignment. This was the Wild West of early networking, where researchers could try new ideas without bureaucratic overhead.
By the time RFC 820 arrived, these range meanings disappeared. The port space became a single unified list of assignments. Port 255 transitioned from "experimental function" to simply "reserved."2
What Reserved Means
Reserved ports are assigned to IANA itself for special purposes. They're not available for regular service assignment. They're not meant to run standard protocols. They exist as placeholders—numbers held back from the general pool.
This is different from "unassigned" ports (like most of the 1024-49151 range), which are simply waiting for someone to register a service. Reserved ports are deliberately kept empty.
Why This Matters
The number 255 appears everywhere in computing:
- Maximum value of an 8-bit unsigned integer (2⁸ - 1 = 255)
- Broadcast address suffix (like 192.168.1.255)
- Subnet mask boundary (255.255.255.255)
But port 255? Empty. The significance of the number makes its absence more noticeable. It's a deliberate pause at the edge of the well-known range.
No Unofficial Uses
Unlike some reserved ports that end up carrying unofficial traffic, port 255 appears genuinely unused. Security scans don't commonly find services listening here. It's not a popular choice for custom applications or malware command-and-control channels. It's just... quiet.
Checking What's Listening
To check if anything is listening on port 255 on your system:
On Linux/Mac:
On Windows:
You'll almost certainly find nothing. Port 255 remains one of the genuinely empty spaces in the port range.
The Architecture of Absence
Port 255 represents something important about Internet architecture: not every number needs to be used. The port space isn't optimized for efficiency—it's optimized for clarity, future flexibility, and the possibility of protocols we haven't imagined yet.
IANA holds port 255 in reserve because they learned that once you assign a port number, it's nearly impossible to take it back. Better to keep some numbers empty than to clutter the well-known range with assignments that might become obsolete.
Related Ports
- Port 0: Another reserved port, used for special purposes in some operating systems
- Ports 224-254: The surrounding numbers in the original experimental range
- Ports 256-1023: The rest of the well-known port range
Frequently Asked Questions
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