Port 3566: Unassigned — An Empty Address in a Vast Space

What Range This Port Belongs To

Port 3566 falls in the registered port range: ports 1024 through 49151. This range sits between the well-known ports (0–1023, reserved for foundational protocols like HTTP, SSH, and DNS) and the ephemeral ports (49152–65535, used temporarily by operating systems for outbound connections).

The registered range exists for a specific purpose: software vendors can formally claim a port number with IANA — the Internet Assigned Numbers Authority — so their applications have a consistent, documented home. MySQL lives at 3306. PostgreSQL at 5432. Redis at 6379. Each of those registrations is intentional, filed, and on record.

Port 3566 is not on that record. IANA lists it as unassigned. No vendor claimed it, no RFC defined a protocol for it, no widely deployed software made it its home.¹

Any Known Unofficial Uses

Research turns up nothing well-established. Some port databases list it as "unassigned" with no additional notes. The DSLReports forums mention UDP port 3566 appearing in scan logs — which most likely reflects automated port scanners probing ranges rather than any specific service running there.²

If you're seeing traffic on port 3566 on your own network, it's worth investigating. It could be:

• A custom internal application using an arbitrary high port
• A piece of software that picked a port without registering it
• A misconfigured service
• Automated scanning traffic from outside

None of those are the same thing. The only way to know is to check.

How to Check What's Listening

On Linux or macOS:

# Show what process is listening on port 3566
sudo lsof -i :3566

# Or with ss (modern replacement for netstat)
sudo ss -tlnp | grep 3566

On Windows:

netstat -ano | findstr :3566

The output includes a process ID (PID). Cross-reference that with Task Manager or tasklist to find the application.

From outside your machine:

# Check if port 3566 is open on a remote host
nmap -p 3566 <hostname-or-ip>

Why Unassigned Ports Matter

The port system has 65,535 slots. The well-known range covers only 1,024 of them. The registered range covers another 48,128. Most of those registered ports are unassigned.

This isn't a problem — it's design. Port space is intentionally large so that the assignment system has room to grow without collision. Every port that's claimed reduces the risk of two applications accidentally fighting over the same number.

Port 3566 being empty means the system is working. There's no service that needed this address. If one ever does, they can file with IANA, and the blank becomes a name.

Until then, it's open space — and open space, on the Internet, is a healthy thing to have.