Port 3648: FJCP — Registered, Then Forgotten

What Port 3648 Is

Port 3648 sits in the registered port range — numbers 1024 through 49151, where organizations and developers can formally stake a claim with IANA to use a port for a specific service.

In 2003, someone at Fujitsu registered this port under the name fjcp: Fujitsu Cooperation Port. That is the entirety of the public record. No RFC. No public specification. No documentation of what "cooperation" refers to, what systems communicate over it, or whether the service was ever broadly deployed.¹

This happens more often than you might expect. The registered port range contains hundreds of entries from the 1990s and 2000s where companies reserved numbers for internal or proprietary protocols that never became public standards.

What to Know If You See It

If port 3648 appears in your network scans, it is almost certainly not FJCP. The registered name is an artifact of history.

More likely explanations:

• A Fujitsu enterprise product using it for internal communication
• An application that picked this number for its own reasons (unassigned-looking ports are common targets for this)
• A misconfigured or legacy service

Check what is actually listening before drawing conclusions.

How to Check What Is Listening

On Linux/macOS:

# Show what process is using port 3648
ss -tlnp | grep 3648
# or
lsof -i :3648

On Windows:

netstat -ano | findstr :3648

Then cross-reference the process ID against your running services.

Why Unassigned-Looking Ports Matter

The registered port range is large — over 48,000 numbers — and IANA does not strictly enforce it. Many registered ports correspond to services that never shipped. Many unregistered ones are used by widely deployed software.

What this means practically: seeing an unfamiliar port number in a network scan tells you almost nothing on its own. The registration database is a starting point, not a source of truth. The only reliable way to know what is on a port is to look at what is running on your machine.

Port 3648 is a useful reminder of this. The name exists in the registry. The protocol, if it ever had one, does not exist anywhere you can read.