Port 3641: Netplay Port 2 — A Registered Ghost

What Port 3641 Is

Port 3641 sits in the registered port range (1024–49151). These ports aren't controlled by IANA the way well-known ports are, but anyone can request an official registration. Once registered, a port gets a name and an entry in the IANA database. In theory, that name follows the port forever.

In theory.

Port 3641 is officially registered as Netplay Port 2, assigned to Predrag Filipovic in November 2002. Its sibling, port 3640, is Netplay Port 1. Both were registered at the same time, presumably for some networked gaming service or protocol that Filipovic was building or planning to build.¹

That's where the trail ends. No RFC. No documentation. No software package with a version history. No forum threads asking why it stopped working. Whatever Netplay Port 2 was for, it left almost no footprint on the Internet.

The Ghost Registration Problem

The IANA registered port registry is full of ports like this — claimed in the early 2000s during a period when registering a port felt like planting a flag, before anyone fully understood that the flag would outlast the settler.

Registrations don't expire. Port 3641 will officially be "Netplay Port 2" in perpetuity, even if nobody has used it that way in two decades. The name is a vestige. The port itself is functionally unoccupied.

This matters because unoccupied registered ports are available real estate. Applications, games, local services, and development environments reach for ports in this range all the time when they need a number that isn't taken in practice. Port 3641 is as available as any unassigned port.

What Might Be Listening Here

If you see traffic on port 3641 on your system, it almost certainly isn't Netplay Port 2. More likely candidates:

• A game or emulator choosing a port in this range for multiplayer sessions
• A local development server that needed a free port
• Custom application infrastructure using this range for internal communication
• Scanning traffic from the Internet probing for open ports (common across all registered ports)

How to Check What's Using This Port

On Linux or macOS:

# Show what process is listening on port 3641
ss -tlnp | grep 3641

# Or with lsof
lsof -i :3641

On Windows:

# Show listening processes
netstat -ano | findstr :3641

# Find the process by PID
Get-Process -Id <PID>

If nothing comes back, nothing is listening. The port is idle.

Why Unoccupied Ports Still Matter

The registered port range exists to prevent collisions. If every application picked port numbers at random, two services on the same machine would inevitably fight over the same port. Registration is how you signal "this is mine" — even if what's yours is a protocol that never shipped.

For port 3641, the registration is more historical artifact than active claim. The port is free to use. Just know what you're running on it.