Port 171: Network Innovations Multiplex — A Ghost in the Registry

The Service That Disappeared

Port 171 is officially assigned to Network Innovations Multiplex, a service that existed long enough to secure an IANA port assignment but not long enough to leave any meaningful trace in Internet history.¹

The assignment is real. The service name appears in the official registry. But if you search for what Network Innovations Multiplex actually did, you find nothing. No documentation. No RFC. No archived manuals. The company is gone. The protocol is gone. Only the port number remains.

What We Know (Which Isn't Much)

Port 171 sits in the well-known ports range (0-1023), which means it was assigned by IANA for a specific standardized service.² These ports were claimed in the early days of networking, when the Internet was small enough that you could just ask Jon Postel for a port number and he'd add it to the list.

Network Innovations apparently built something called "Multiplex" that needed its own port. They got port 171. That's about all the history that survived.

There's a related assignment on port 172 for "cl/1" (Network Innovations CL/1), which IANA now marks as historic. The "cl-1" service name was created as a well-formed replacement for "cl/1" because the slash character breaks modern service discovery mechanisms.¹ This suggests Network Innovations had at least two related services, and that they were old enough to predate naming conventions.

The Nature of Obsolete Ports

Port 171 represents something important about Internet infrastructure: most of it dies without ceremony.

The well-known ports range is filled with assignments like this—services that seemed important enough to deserve a permanent port number, then disappeared. The port assignments remain because removing them would be more complicated than leaving them. They're ghosts in the registry.

You won't find anything listening on port 171 on modern systems. The service is extinct. But the port is still officially "taken," which means it can't be reassigned to something else. It's reserved for a service that no longer exists.

Why This Matters

These forgotten ports tell you something about how the Internet evolved:

Early assignments were permanent — Once you got a well-known port, you kept it. There was no mechanism for reclaiming ports from dead services.

Most protocols die — For every HTTP (port 80) that becomes universal, there are dozens of services like Network Innovations Multiplex that vanished completely.

The registry is archaeological — Looking through IANA's port assignments is like reading a cemetery. Most entries are tombstones for protocols nobody uses anymore.

What's Probably Listening (Nothing)

If you scan port 171 on a modern system, you'll almost certainly find nothing. The service is extinct.

If you do find something listening on port 171, it's either:

• A custom application using an unoccupied port
• Malware specifically chosen an obscure port to hide
• An incredibly old system still running Network Innovations software (unlikely bordering on impossible)

How to Check Your System

On Linux/Mac:

# See if anything is listening on port 171
sudo lsof -i :171
# Or using netstat
netstat -an | grep :171

On Windows:

# Check for listeners on port 171
netstat -an | findstr :171

If you find nothing, that's correct. Port 171 is a historical artifact, not an active service.

Related Ports

• Port 172 — Network Innovations CL/1 (also historic, also forgotten)
• Other obsolete well-known ports scattered throughout 0-1023, each with their own forgotten story