Port 3506: APC Power Management — A Registered Port with a Quiet Owner

What This Port Is

Port 3506 sits in the registered port range (1024–49151), which IANA manages for services that have formally claimed a number. The registrant here is APC — American Power Conversion, the company behind the UPS (Uninterruptible Power Supply) units that live under server room floors and in data center racks worldwide. APC is now part of Schneider Electric.

The service name in the IANA registry is apc-3506. Documentation beyond the registration itself is sparse. APC's network management software suite (PowerChute and related tools) uses various ports for device communication, monitoring, and remote management, and this port appears to be part of that family — though it's not the primary port APC software uses day-to-day.

What the Registered Range Means

The registered port range (1024–49151) is the middle tier of the port numbering system:

• 0–1023: Well-known ports. Assigned to foundational Internet services (HTTP, SSH, DNS, SMTP). Require root/admin privileges to bind on most systems.
• 1024–49151: Registered ports. Applications and vendors claim these through IANA. No privilege requirement to bind, but numbers are reserved on paper.
• 49152–65535: Dynamic/ephemeral ports. Used temporarily by operating systems for outbound connections. Not registered.

Having a registered port doesn't mean a service is actively listening on your machine. It means someone filed the paperwork. Most systems never run apc-3506.

Observed in the Wild

The SANS Internet Storm Center shows regular scanning activity on port 3506 — automated probes from external IP addresses checking whether anything responds. This is unremarkable. Scanners sweep registered ports looking for misconfigured or exposed services. If you're not running APC network management software, nothing should be listening here, and those probes will go unanswered.

One additional use: the game Bet on Soldier: Blood Sports by Take-Two reportedly used UDP port 3506 for multiplayer connectivity. Whether that was intentional reuse or coincidence isn't documented, and the game is long discontinued.

How to Check What's Listening

If you want to verify nothing unexpected is bound to this port on your system:

macOS / Linux:

# Show processes listening on port 3506
lsof -i :3506

# Or with ss (Linux):
ss -tlnp | grep 3506

Windows:

netstat -ano | findstr :3506

An empty result is the expected result. If something is listening and you didn't put it there, that's worth investigating.

Why Unguarded Registered Ports Matter

The registered port range is large enough that most of it sits quietly unused on any given machine. But "unoccupied" and "safe to ignore" aren't the same thing. Firewalls should be explicit: allow what you need, deny everything else. Relying on "nothing should be using that port" as a security posture is how surprises happen.

Port 3506 specifically: if you're not running APC network management software, there's no reason this port should be reachable from the Internet.