Port 3573: tag-ups-1 — A Registered Port Nobody Uses

What Port 3573 Is

Port 3573 is a registered port — officially assigned by IANA to a service called tag-ups-1, the network component of the Advantage Group UPS Suite.¹

The Advantage Group was a St. Louis, Missouri company founded in 2000 that built hardware and software for managing Uninterruptible Power Supplies (UPS devices — the battery backups that keep servers running when power fails). In 2002, they registered port 3573 with IANA as part of their Device Manager software suite, which could monitor thousands of UPS units across an enterprise network.²

The company has since faded from prominence. The software is no longer in active development, the port sees essentially no traffic in practice, and most port scanners will find nothing listening here.

The Registered Port Range

Port 3573 sits in the registered ports range, which spans 1024 to 49151.

Registered ports (also called user ports) are assigned by IANA upon application from a vendor or standards body. Unlike the well-known ports (0–1023), which require root/administrator privileges to bind, registered ports can be used by ordinary applications. The registration is intended to prevent conflicts — if two companies both want port 3573, IANA's assignment system resolves the dispute.

The catch: IANA cannot enforce what actually runs on these ports. Registration reserves a name in a registry, not a port on your machine. Port 3573 is "claimed" by the Advantage Group in the same way a company might trademark a name — it means something legally and organizationally, but nothing technically stops other software from using it.

What You'll Actually Find on Port 3573

Almost certainly nothing.

If your network scanner reports activity on port 3573, it is not the Advantage Group UPS Suite. More likely candidates:

• Custom internal applications that happened to pick this port
• Development servers assigned a random high port
• Malware or port scanners probing your system

The UPS Suite never achieved widespread deployment. Even in 2002, the market for enterprise UPS management software was competitive and specialized. Today, open-source alternatives like Network UPS Tools (NUT)³ have largely absorbed the use cases this software targeted — and NUT uses port 3493, not 3573.

How to Check What's Listening

On Linux/macOS:

# Show what process is using port 3573
ss -tlnp | grep 3573

# Or with lsof
lsof -i :3573

On Windows:

# Show listening ports with process IDs
netstat -ano | findstr :3573

# Match the PID to a process
tasklist | findstr <PID>

If these commands return nothing, nothing is listening. That's the expected result.

Why Unassigned-in-Practice Ports Matter

Port 3573 illustrates something real about how the port system works: registration and usage are entirely separate things.

IANA's registry has over 49,000 registered port entries. Many of them point to software that no longer ships, companies that no longer exist, or protocols that were superseded before they gained adoption. The registry is a historical record as much as an operational one.

This matters for network security. When a firewall rule blocks "all unregistered ports," it doesn't actually mean much — registered ports are not trusted ports. It matters for debugging too: finding an unfamiliar port number in a registry entry is not the same as finding out what's actually running on your system.

The authoritative answer to "what is using port 3573?" is always the same: look at the machine itself.