Port 235: Reserved — A Port Held in Reserve

Port 235 is reserved by the Internet Assigned Numbers Authority (IANA). It has no assigned service, no protocol, and no defined purpose. It's not unassigned through oversight - it's intentionally held back for potential future use.¹

What Reserved Means

Port 235 sits in a block of 17 ports (225-241) that are collectively marked as "Reserved" in the IANA registry.² This is different from unassigned ports:

• Unassigned ports are open for applications through IANA's assignment process
• Reserved ports are held back by IANA for specific future purposes
• Assigned ports have a defined protocol and service name

Reserved ports exist in a liminal state - they're protected from casual use but have no defined function. IANA holds them for protocols that might need them someday, or for reasons lost to time.

The Well-Known Port Range

Port 235 falls within the well-known ports range (0-1023), also called system ports.³ These ports are special:

• On Unix-like systems, binding to these ports requires superuser privileges⁴
• They're assigned by IETF Review or IESG Approval - not first-come, first-served⁵
• They're meant for fundamental Internet services (HTTP, SSH, DNS, etc.)

The well-known ports were originally numbered 0-255 when the Internet was young. As the network grew, the range expanded to 0-1023.⁶ Port 235 has been reserved since this system was formalized - held in reserve for four decades, waiting for a purpose that has never arrived.

Why Ports Get Reserved

IANA reserves ports for several reasons:

• Future protocols that might need specific port numbers
• Historical conflicts that make a port unsuitable for general assignment
• Administrative purposes that require holding certain ranges
• Legacy decisions where the original reasoning is unclear

In the case of ports 225-241, the collective reservation suggests they were set aside as a block, possibly for a protocol suite that needed multiple adjacent ports or for organizational purposes that are no longer documented.

Checking What's Listening

Even though port 235 is reserved and has no official service, something could still be listening on it on your system. Here's how to check:

On Linux/macOS:

# Check what's listening on port 235
sudo lsof -i :235

# Or using netstat
sudo netstat -tulpn | grep :235

On Windows:

# Check what's listening on port 235
netstat -ano | findstr :235

If you find something listening on port 235, it's either a misconfigured service or an application using reserved ports for private purposes.

The Beauty of Reserved Space

There's something remarkable about reserved ports. They're not forgotten or abandoned - they're intentionally held back, like seats at a theater for guests who haven't yet arrived. Port 235 has been waiting for 40+ years for a protocol important enough to claim it.

Most reserved ports will never be used. The Internet has moved on to dynamic port allocation for most services, and the well-known port range is nearly full of assigned services. But IANA keeps these ports in reserve anyway, because you never know when you might need a permanent, privileged port for something that matters.

Port 235 is a bookmark in the Internet's address space - a reminder that not everything needs to be used immediately to have value.

Security Considerations

Reserved ports should not be accessible from the Internet on most systems:

• No legitimate service should be listening on port 235
• If you see traffic on this port, investigate it
• Firewalls should typically block unassigned/reserved well-known ports
• Some network scanning tools check reserved ports looking for misconfigurations

Related Ports

Port 235 sits in the reserved block 225-241. Nearby ports include:

• Port 220 - IMAPv3 (historical, superseded by IMAP4)
• Port 443 - HTTPS (the secure web)
• Port 989-990 - FTPS (FTP over SSL/TLS)