Port 3640: Netplay Port 1 — A Name Without a Protocol

What Port 3640 Is

Port 3640 sits in the registered port range (1024–49151). These ports require an application to IANA for assignment — unlike the well-known ports below 1024, which are reserved for foundational protocols, or the ephemeral ports above 49151, which operating systems assign dynamically to outgoing connections.

IANA's registry lists port 3640 as netplay-port1, with port 3641 alongside it as netplay-port2. Both TCP and UDP are noted. That's where the official record ends.

No RFC defines what "netplay" means here. No widely distributed software ships with these ports. No security advisories track them. The name was registered — and then the protocol, if one was ever planned, never materialized.

What "Registered but Unused" Means

Registration is not the same as deployment. Anyone can apply to IANA to reserve a port number for a service they intend to build. Sometimes those services ship and become essential infrastructure. Sometimes they don't. Port 3640 is in the second category.

The registered port range contains thousands of entries in varying states of vitality: actively used protocols, niche enterprise software, and quiet placeholders like this one. The registry is a historical record as much as it is a map of the living Internet.

What Might Actually Be on Port 3640

If port 3640 shows up on a machine you're investigating, it has nothing to do with the IANA label. It's likely:

• Application-specific traffic — Many applications pick ports in the registered range without formal registration. Proprietary software, development servers, and internal tools frequently land here.
• A scan probe or false positive — Security scanners sometimes report ports based on registry labels rather than observed traffic.
• Something worth investigating — An open port with no obvious owner deserves a closer look.

How to Check What's Listening

On Linux or macOS:

# Show which process is listening on port 3640
sudo ss -tlnp | grep 3640

# Or with lsof
sudo lsof -i :3640

On Windows:

netstat -ano | findstr :3640

The output will show the process ID. Cross-reference with Task Manager or ps aux to identify what's actually running.

Why Unassigned Ports Matter

Port numbers are a namespace, and namespaces get cluttered. The registered range has over 48,000 possible ports, and many are either loosely documented, historically assigned to defunct software, or sitting like port 3640 — named, but not inhabited.

This matters for security. An unexpected open port on a server isn't less interesting because IANA assigned it an obscure label. If you see 3640 open and you didn't put it there, find out what's listening before you assume it's benign.