Port 2566: pcs-pcw — A Reserved Name With No Explanation

What This Port Is

Port 2566 sits in the registered port range (1024–49151), the middle tier of the port numbering system. These ports require an application to IANA, which assigns the name and records it in the registry. Unlike well-known ports (0–1023), registered ports don't require elevated privileges to use—any process can bind to them.

IANA lists port 2566 under the service name pcs-pcw, on both TCP and UDP. That's where the documentation ends.

The Ghost in the Registry

"pcs-pcw" appears in IANA's official port registry but has almost no presence elsewhere. No RFC was written for it. No vendor has publicly documented it. No major application lists it as a dependency. Port reference databases that mention it simply echo the IANA entry without adding anything.

This is not unusual. The registered port range has thousands of entries like this—ports requested by organizations or developers who either never shipped the software, quietly moved to a different port, or simply stopped maintaining any public presence. IANA assigns the name and moves on; there's no requirement to publish documentation or keep a contact updated.

What "pcs-pcw" stands for is genuinely unclear. "PCS" could refer to Personal Communications Service, a physical coding sublayer, or something entirely proprietary. "PCW" adds no obvious disambiguation. Without the original registrant, the name is an artifact without context.

Security Notes

Some port databases flag port 2566 as having historical associations with malicious traffic.¹ This is a weak signal—many ports carry these warnings, and it typically means someone, somewhere, once observed a malware sample communicating on this port. It does not mean port 2566 is inherently dangerous or that legitimate traffic on it is suspect.

If you see unexpected traffic on port 2566, the right response is to identify what process is using it, not to assume the worst.

How to Check What's Listening

If port 2566 is active on your system, these commands will tell you what's using it:

Linux / macOS:

# Show the process bound to port 2566
lsof -i :2566

# Alternative using ss
ss -tlnp | grep 2566

Windows:

netstat -ano | findstr :2566

The PID returned by these commands can be matched against your process list (ps aux on Linux/macOS, Task Manager on Windows) to identify the application.

Why Unassigned Ports Matter

Ports like 2566 serve a quiet structural function. The registered port range exists so that applications can stake out a consistent home—a port they can advertise to users and firewall administrators without stepping on well-known protocols. When the registration is maintained and documented, this works well. When it isn't, you get ghost entries like pcs-pcw: a reservation with no tenant.

For administrators, this matters because firewall rules and monitoring systems that block or alert on "unknown" ports are making decisions in exactly this gray zone. An unassigned port with traffic isn't necessarily suspicious—it might be a legitimate application that simply never bothered to register, or one that registered under a name no one can trace anymore.

• IANA Service Name and Transport Protocol Port Number Registry
• RFC 6335 — IANA Procedures for Port Number Registry