Port 1600: ISSD — The protocol that never found its audience

Port 1600 is officially registered for ISSD (short for what appears to be a display management protocol), operating on both TCP and UDP. ISSD is described as a text-based protocol designed for managing and controlling remote system displays—allowing administrators to interact with and configure displays on devices over a network.¹²

There's just one problem: almost nobody uses it.

What Is ISSD?

ISSD was registered with IANA as the official service for port 1600. The protocol was intended to provide remote display management capabilities—think of it as a way to control what appears on a screen from somewhere else on the network.

The protocol operates over both TCP (for reliable connections) and UDP (for lightweight messaging), giving it flexibility in how it communicates. The text-based nature suggests it uses human-readable commands rather than binary protocols.

But despite having an official port assignment and a documented purpose, ISSD never became a standard tool in network administration. Display management evolved in different directions—VNC, RDP, X11 forwarding, modern remote desktop protocols—and ISSD remained a footnote in the registry.

The Registered Ports Range

Port 1600 sits in the registered ports range (1024-49151). This is the middle tier of the port system:³

• Well-known ports (0-1023): Reserved for core Internet services like HTTP, HTTPS, SSH
• Registered ports (1024-49151): Registered with IANA for specific applications or services
• Dynamic/ephemeral ports (49152-65535): Used temporarily by client applications

The registered range contains thousands of port assignments. Some are household names (MySQL on 3306, PostgreSQL on 5432). Others, like ISSD on 1600, are officially registered but rarely seen in practice.

Why Unassigned and Obscure Ports Matter

The existence of ports like 1600 tells us something important about the Internet's architecture: the system was built with room to grow.

IANA doesn't just assign ports to proven, popular protocols. It assigns them to ideas—to protocols someone thought might be useful someday. Most of those ideas never take off. The registered range is full of protocols that sounded promising in a proposal but never found an audience.

This isn't a failure of the system. It's a feature. The Internet needed a way to say "yes, try your idea" without knowing which ideas would succeed. Port registration is cheap. Implementation and adoption are hard.

Port 1600 has an official assignment, but that doesn't mean anything is actually listening there on most systems. It's a door that exists but rarely opens.

Security Considerations

Precisely because port 1600 is rarely used for its intended purpose, finding it open on a system should raise questions:

• Is something actually using ISSD? Unlikely, unless you're working with very specific legacy equipment.
• Is it a misconfiguración? Something else might have bound to 1600 unintentionally.
• Is it malicious? Malware occasionally uses obscure registered ports because they look legitimate but aren't monitored.

The general security principle: if you don't know why a port is open, find out. Especially for ports in the registered range that aren't commonly used.

Checking What's Listening on Port 1600

On Linux or macOS:

# See what's listening on port 1600
sudo lsof -i :1600

# Or using netstat
netstat -an | grep 1600

On Windows:

# Check port 1600
netstat -ano | findstr :1600

If you find something listening on port 1600 and you don't recognize the process, investigate. It's probably not ISSD.

The Story Behind the Numbers

Every port number represents someone's idea of how computers should communicate. Port 22 exists because Tatu Ylönen wanted secure remote login. Port 80 exists because Tim Berners-Lee needed a way to serve web pages. Port 1600 exists because someone thought remote display management needed a dedicated protocol.

Most ideas don't change the world. Most protocols don't get adopted. The registered ports range is a graveyard of good intentions—thousands of port assignments for protocols that never found their moment.

But the system works precisely because it accommodates failure. IANA doesn't need to predict winners. It just needs to make room for experiments. Port 1600 got its chance. ISSD didn't become the standard. Something else did.

That's how progress works: lots of tries, most failures, a few world-changing successes. The port registry preserves the evidence of every attempt.

Related Ports

• Port 5900-5901 — VNC (Virtual Network Computing), a widely-used remote display protocol
• Port 3389 — RDP (Remote Desktop Protocol), Microsoft's remote display standard
• Port 6000-6063 — X11, the Unix/Linux graphical display protocol
• Port 177 — XDMCP (X Display Manager Control Protocol), another display management protocol

These ports represent the protocols that actually solved the remote display problem. ISSD on port 1600 remains the road not taken.