Port 253: Reserved — A Boundary Held Back

Port 253 is reserved by IANA. It sits in a block of seven consecutive ports (249-255) that have been marked "Reserved" since RFC 1060 in 1990.¹² No service runs here officially. No protocol claims it. IANA holds it for reasons that may be structural, historical, or simply precautionary.

What "Reserved" Means

In IANA's port registry, there are three states a port can occupy:

• Unassigned — Available for assignment upon request
• Assigned — Allocated to a specific service or protocol
• Reserved — Held by IANA, not available for regular assignment³

Port 253 is in the third category. Reserved ports exist for special purposes: boundary markers, future expansion, or administrative control. According to RFC 6335, reserved port numbers "are not available for regular assignment" and are "assigned to IANA" for potential future use.³

Why Ports 249-255 Are Reserved

The specific reason these seven ports were reserved is not explicitly documented. Port 253 sits just before port 256—the start of the next "block" in what was originally an 8-bit addressing boundary. The well-known ports range (0-1023) was defined when the Internet was young, and certain boundaries were preserved.⁴

RFC 1060, published in March 1990, lists ports 249-255 as "Reserved."⁴ At that time, most assigned ports fell below 256. The decision to reserve this particular range suggests it may have been held back for structural reasons related to the port numbering architecture, or simply as a buffer zone before the next numerical boundary.

The reservation has persisted for over three decades. The IANA registry was updated as recently as December 2024, but the reserved status remains unchanged.²

The Well-Known Ports Range

Port 253 falls within the well-known ports range (0-1023), also called system ports. These are assigned by IANA through rigorous processes—typically requiring IETF Review or IESG Approval.³

Getting a well-known port assigned requires meeting strict criteria. These ports are for protocols and services that need universal recognition across the Internet. Reserved ports like 253 sit in this valuable range but remain intentionally unallocated.

What Actually Listens on Port 253

On most systems, nothing. If you scan port 253 on your machine or across the Internet, you'll typically find it closed or filtered. Because no official service is assigned to it, legitimate software doesn't use it.

However, that doesn't mean it's never used. Malware, custom applications, or unofficial services can bind to any port, including reserved ones. If something is listening on port 253, it's non-standard.

How to Check Port 253

On Linux or macOS:

# See if anything is listening on port 253
sudo lsof -i :253

# Or using netstat
netstat -an | grep 253

On Windows:

# Check if port 253 is in use
netstat -an | findstr :253

If you find something listening, investigate what process owns it. Reserved ports should typically remain unused.

Why Unassigned and Reserved Ports Matter

The Internet's port system (0-65535) is a finite resource. Reserved ports like 253 represent conscious decisions to hold back certain numbers for potential future needs. They're part of the infrastructure's capacity planning—space held in reserve.

Unassigned ports can be requested and allocated. Reserved ports cannot. This distinction matters because it affects what's available for new protocols and services as the Internet evolves.

The 249-255 range has remained reserved for 34 years. Whether it will ever be allocated, or what it might be used for, remains an open question.