Port 172: Network Innovations CL/1 — The Ghost in the Registry

Port 172 sits in the well-known ports range (0-1023), the territory reserved for fundamental Internet services. But nothing fundamental happens here anymore.

What Lives Here (Or Doesn't)

Port 172 was officially assigned to Network Innovations CL/1, a protocol designated by the service name "cl-1" in the IANA registry.¹ Both TCP and UDP port 172 were allocated for this service, with Kevin DeVault listed as the contact person.²

The protocol name suggests it was related to port 171, which was assigned to "Network Innovations Multiplex"—apparently this company had adjacent ports for related services. But here's the catch: IANA now marks this entry as historic, which is the registry's polite way of saying "this protocol is dead and we're keeping the tombstone for the records."¹

Historic means the assignment is "not usable for use with many common service discovery mechanisms." Translation: don't expect any modern software to know what to do with port 172.

The Well-Known Ports Range

Port 172 occupies space in the well-known ports range (0-1023), sometimes called system ports. For many years, assigned ports only went up to 255, but IANA eventually expanded the managed range to 1023.³

These ports require special privileges to bind to on Unix-like systems—you need root access to listen on port 172. That protection exists because these low-numbered ports were meant for essential system services. Services like HTTP on 80, HTTPS on 443, SSH on 22.

Port 172 was supposed to be one of those essential services. It wasn't.

What Happened to Network Innovations

Nobody knows. The company is gone. The protocol is gone. The service discovery mechanisms don't recognize it. All that remains is the registry entry, marked historic, and a port number that still technically belongs to something that no longer exists.

This is the Internet's version of abandoned architecture—foundations poured for buildings that were never finished.

Conservation Through De-assignment

IANA has procedures for de-assigning port numbers, allowing previously assigned ports to become unassigned through revocation or transfer to new applications.⁴ Port 172 hasn't been de-assigned; it's just been marked historic and left alone.

Some ports get fully revoked and reassigned. TCP port 465, for example, was originally assigned for SMTP over SSL, left unused, revoked, and then re-assigned to Cisco's URD protocol.⁴ Port 172 hasn't gotten that treatment. It's just marked historic and forgotten.

When approximately 76% of the well-known ports are assigned⁴, there's room to let the dead ones rest.

Checking What's Listening

Even though port 172 has no active official service, you can still check if anything is listening on it on your system:

# Check if anything is listening on port 172
sudo lsof -i :172

# Or using netstat
sudo netstat -tlnp | grep :172

# On Windows
netstat -ano | findstr :172

If something is listening on port 172, it's either:

• Custom software that chose an obscure port
• Malware hiding in an unused port number
• A very old piece of Network Innovations software (extremely unlikely)

Why This Port Matters

Port 172 matters precisely because it doesn't matter anymore. It's a reminder that the Internet is built on layers of abandoned infrastructure—protocols that seemed important enough to get well-known port assignments but weren't important enough to survive.

The IANA registry is full of these ghosts. Historic assignments. Obsolete RFCs. Port numbers pointing to companies that no longer exist and protocols nobody remembers.⁴

Every port number in the well-known range is valuable—there are only 1024 of them, and they require root privileges to use. Port 172 is one of those valuable numbers, sitting empty, marked historic, waiting for nothing.

It's honest. Most things we build don't last. Most protocols don't become HTTP or SSH. Most companies don't become institutions. Port 172 is the Internet admitting that sometimes we allocate resources for things that simply fade away.