Port 20039: Unassigned — The Second User's Door

Port 20039 has no officially assigned service. IANA lists it in the registered ports range (1024-49151) as unassigned. But search the web and you'll find traces of what it used to carry: ICQ instant messages for the second user behind a firewall.

What the Registered Range Means

Ports are divided into three ranges:

• Well-known ports (0-1023): Reserved for system services, assigned by IANA
• Registered ports (1024-49151): Registered with IANA for specific services, but not reserved
• Dynamic/private ports (49152-65535): Never assigned, used for temporary connections

Port 20039 sits in the registered range. Anyone can use it, but IANA maintains a registry to prevent conflicts. This port has no entry in that registry. It's officially unassigned.

The ICQ Era: Why This Port Existed

In the late 1990s and early 2000s, ICQ was how people talked online. The protocol used UDP port 4000 for client-to-server communication and port 5190 for the OSCAR protocol. But if you were behind a firewall and wanted to receive direct connections—for file transfers, chat requests, incoming messages—you needed to configure TCP listening ports.¹

ICQ divided these ports into ranges:

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

Port 20039 was the last port in the second user's range. If two people in a household both used ICQ behind the same firewall, one would configure ports 20000-20019, and the other would use 20020-20039. You'd open these ports on your firewall, configure ICQ to listen on them, and incoming connections would reach you.²

Why This Matters (And Why It Doesn't)

This port represents a problem that no longer exists. Modern NAT handles multiple users automatically. Instant messaging moved to centralized services that don't require port forwarding. ICQ itself shut down in 2024.³

But port 20039 remains. Unassigned. Available. A door with no service behind it.

What Could Be Listening on Port 20039 Today

Since the port is unassigned, anything could be using it:

• A custom application
• Malware (unassigned ports are sometimes exploited)
• A developer testing something
• Nothing

To check what's listening on this port on your system:

# Linux/Mac
sudo lsof -i :20039
netstat -an | grep 20039

# Windows
netstat -an | findstr "20039"

If something is listening and you don't recognize it, investigate.

The Value of Unassigned Ports

The registered range contains thousands of unassigned ports. They exist as open space—room for new protocols, new services, new ways of connecting. Port 20039 once carried ICQ messages for the second user in a household. Tomorrow it could carry something else entirely.

The Internet's port system is a 65,535-room building. Some rooms have permanent residents. Some are reserved but empty. Port 20039 is a room with history but no current tenant. The door is unlocked. The space is available. What happens next is up to whoever needs it.