Port 20040: ICQ's Third User — The Port That Remembers When Sharing Mattered

Port 20040 lives in the registered ports range (1024-49151), that middle territory between the well-known ports that run the Internet's core services and the ephemeral ports your computer assigns randomly. Registered ports are assigned by IANA upon request, but many see unofficial use—and port 20040's story is one of those uses.

What This Port Does

Port 20040 was part of ICQ's port range allocation for users behind firewalls or proxies. Specifically, it was the first port in a 20-port range (20040-20059) designated for the third user's incoming events.¹

The scheme worked like this:

• First user: ports 20000-20019
• Second user: ports 20020-20039
• Third user: ports 20040-20059

If you were the third person in your household trying to use ICQ, you'd configure your client to listen on these ports for incoming messages and file transfers.

Why This Existed

In the late 1990s and early 2000s, Network Address Translation (NAT) wasn't as sophisticated as it is today. If multiple people behind the same router wanted to use ICQ simultaneously, they couldn't all use the same ports—the router wouldn't know which person to send incoming data to.

The solution was manual port allocation. Each user would configure their ICQ client with a different port range, then configure the router to forward those specific ports to their specific computer. It was tedious. It required understanding firewalls. But it worked.

Port 20040 exists because someone was the third person who wanted to chat.

The Obsolescence

Modern ICQ clients (before the service shut down in 2024) didn't require this manual configuration. NAT traversal techniques, STUN servers, and smarter protocols solved the multiple-user problem automatically. But for years, port 20040 and its siblings were how families coordinated their Internet presence.²

Security Considerations

If you find port 20040 open on a modern system, it's either:

• A legacy ICQ installation (increasingly rare)
• A custom application using this port unofficially
• A misconfiguration left over from the 2000s

There's no official IANA assignment for port 20040, so any service running here is doing so without formal registration.³

How to Check What's Listening

# On Linux/Mac
sudo lsof -i :20040
netstat -an | grep 20040

# On Windows  
netstat -ano | findstr :20040

The Registered Ports Range

Port 20040 sits in the registered ports range (1024-49151). These ports are:

• Assignable by IANA for specific services
• Usable without superuser privileges on most systems
• Often used informally by applications without official registration

Unlike well-known ports (0-1023) which require root access and are reserved for fundamental Internet services, registered ports are the middle ground—official enough to coordinate, informal enough to experiment.

Why Unassigned Ports Matter

The Internet has 65,535 possible ports. Only a fraction have official assignments. The unassigned space is where innovation happens—where developers can run custom services, where organizations can deploy internal applications, where the next protocol can be tested before it becomes standard.

Port 20040 is part of that space. It carried ICQ messages for millions of third users. Now it's available for whatever comes next.

Related Ports

• Ports 20000-20019: ICQ first user incoming events
• Ports 20020-20039: ICQ second user incoming events
• Port 5190: ICQ's main client-to-server connection (OSCAR protocol)
• Ports 49152-65535: Ephemeral/dynamic ports, assigned automatically by the OS