Port 256: Reserved — A Number Held in Reserve

Port 256 is reserved by the Internet Assigned Numbers Authority (IANA). It sits in the well-known ports range (0-1023), but unlike ports that carry SSH or HTTPS or DNS traffic, port 256 doesn't do anything. It's not assigned to a service. It's held in reserve.

What "Reserved" Means

In IANA's port number registry, there are three states a port can exist in:¹

• Assigned — Currently allocated to a specific service
• Unassigned — Available for assignment upon request
• Reserved — Not available for regular assignment; held by IANA for special purposes

Port 256 falls into that third category. Reserved ports exist at the edges of port ranges, serve as structural markers, or are set aside for potential future use. When you see "Reserved" in the registry, it means: this number is off-limits for now, for reasons that might be administrative, technical, or simply organizational.

The Well-Known Ports Range

Port 256 lives in the system ports range (0-1023), also called the well-known ports.² These are the ports that typically require root or administrator privileges to bind to on Unix-like systems. They're the foundation of Internet infrastructure—this is where you find HTTP (80), HTTPS (443), SSH (22), and DNS (53).

Being in this range means port 256 was considered significant enough to warrant protection, even if it doesn't currently host a service. The well-known ports are managed through "IETF Review" or "IESG Approval" procedures—formal processes that ensure only legitimate, stable protocols claim these valuable low numbers.³

Why Reserve a Port?

IANA reserves ports for several reasons:

Range Boundaries — Ports at the edges of ranges (like 0, 1023, 1024) are often reserved to maintain clean boundaries or allow for future expansion of those ranges.¹

Protocol Symmetry — When a port is assigned for only TCP or only UDP, the corresponding number in the other protocol is typically marked as reserved to prevent confusion.¹

Historical or Administrative Reasons — Sometimes ports are reserved due to legacy assignments, security considerations, or administrative needs that aren't publicly documented.

Port 256 specifically appears to fall into this category—a number held in reserve as part of the structural organization of the port number space.

What You'll Find Here

Nothing. If you scan port 256 on a typical system, you won't find anything listening. You won't see a service banner, won't get a connection, won't trigger a response. Reserved ports are quiet by design.

You can check yourself with these commands:

On Linux or macOS:

# Check if anything is listening on port 256
sudo lsof -i :256

# Or using netstat
sudo netstat -tulpn | grep ':256'

On Windows:

# Check listening ports
netstat -ano | findstr ":256"

In most cases, you'll find nothing—which is exactly what's expected for a reserved port.⁴

A Note on Port 256 "RAP" References

You may find older documentation or port databases claiming port 256 is assigned to "RAP" (sometimes listed as "Route Access Protocol" or "Remote Authentication Protocol"). This information appears in historical sources but is not reflected in current IANA registry practices. The actual Internet Route Access Protocol (RAP) defined in RFC 1476 uses port 38, not port 256.⁵

The confusion likely stems from outdated registry entries or unofficial port databases that haven't been updated to reflect current IANA status. When researching ports, always verify against the official IANA Service Name and Transport Protocol Port Number Registry.

Why Reserved Ports Matter

Reserved ports might seem like wasted space—numbers sitting idle in a finite address range. But they serve an important structural function. They provide:

Clean Architecture — Clear boundaries between different port ranges help maintain the organizational logic of the Internet's addressing system.

Future Flexibility — Reserved numbers can be allocated when legitimate needs arise, without disrupting existing services.

Administrative Clarity — Having explicitly reserved ports prevents accidental assignments and reduces confusion in the registry.

Think of reserved ports as the margins in a book, or the silence between musical notes. The empty space serves a purpose. It creates structure, prevents collision, and leaves room for what might need to exist later.

The Quiet Ports

Port 256 sits in the registry not as a working door, but as a marker. A number held back from the chaos of assignment. It exists in documentation, in the IANA database, in the complete maps of Internet infrastructure—but not in active use.

There's something oddly peaceful about reserved ports. They're not fighting for attention, not carrying traffic, not vulnerable to exploits. Just numbers in a list, held in reserve like blank pages in a book that might someday tell a story—or might not.

And that's okay. Not every port needs to carry the weight of the Internet. Some can simply exist as structure, as boundary, as possibility held in reserve.