Port 3665: Enterprise Engine Port — Registered but forgotten

What Range This Port Belongs To

Port 3665 falls in the registered ports range: 1024 through 49151. These ports sit between the well-known ports (0–1023, reserved for major protocols like HTTP, SSH, and DNS) and the ephemeral ports (49152–65535, used temporarily by operating systems for outbound connections).

Registered ports don't require elevated privileges to use on most systems. Any application can bind to them. IANA maintains a registry of registered ports to reduce collisions between software vendors — but registration is voluntary, and enforcement is nonexistent. If your application wants to use port 3665, nothing stops it.

What IANA Says

The IANA registry lists port 3665 (TCP) as assigned to "Enterprise Engine Port", registered in January 2003 by a contact named Mike Delgrosso.¹ That's the full extent of the public record.

No RFC. No protocol specification. No vendor documentation. No forum posts from users asking why their firewall is blocking it. The name suggests enterprise software of some kind — a management agent, a job scheduler, a middleware component — but the actual product behind the registration has left almost no trace on the public Internet.

This is not unusual. The registered port range contains hundreds of entries like this one: claimed during a product's development, assigned by IANA, and then quietly orphaned when the software never shipped broadly, was discontinued, or simply never needed to be documented publicly.

Any Known Unofficial Uses

None identified. Port 3665 does not appear in common malware signatures, firewall rule databases, or network monitoring tools as a port of interest. If something is listening on this port on your system, it's almost certainly application-specific rather than a widely recognized service.

How to Check What's Listening

If you see activity on port 3665 and want to know what's using it:

On Linux or macOS:

sudo ss -tlnp | grep 3665
# or
sudo lsof -i :3665

On Windows:

netstat -ano | findstr :3665
# Then look up the PID:
tasklist | findstr <PID>

These commands show which process has bound to the port. From there, the process name usually tells you everything you need to know.

Why Unassigned-in-Practice Ports Matter

The port registry works on trust and convention. IANA assigns numbers; vendors agree to use them. When that agreement breaks down — when a vendor registers a port and the product disappears — the port number becomes a kind of dead letter. It's technically claimed but practically available.

This creates a small problem: software that needs a port might pick 3665 because nothing seems to be using it, without realizing it's registered. Conflicts are rare, but they happen. A port scanner seeing activity on 3665 has no standard service to compare it against, which makes anomaly detection slightly harder.

Port 3665 is harmless and unremarkable. It's a reminder that the registry is a historical document as much as a living standard — a record of software that wanted a number, whether or not it ever became something people used.