Port 3516: Smartcard Port — Registered but Forgotten

What This Port Is

Port 3516 sits in the registered port range (1024–49151). These ports are officially tracked by the Internet Assigned Numbers Authority (IANA) — vendors and developers can reserve them for specific services, distinguishing their protocols from ad hoc assignments.

IANA lists port 3516 as "Smartcard Port", registered in May 2002 and attributed to Scott Guthery, a researcher who worked on smartcard-to-Internet connectivity. ¹ The concept: give smartcards a direct TCP/IP presence, so a card inserted into a reader could be addressed over a network like any other device.

Both TCP and UDP are listed in the IANA registry for this port.

What Actually Happened

The registered port range has a problem: registration is easy, deployment is hard.

The smartcard networking vision from 2002 never achieved mainstream adoption. PC/SC (Personal Computer/Smart Card), the dominant framework for smartcard communication on Windows, macOS, and Linux, uses local inter-process communication — not TCP/IP ports. Smartcard readers don't open network sockets; they expose an API that applications call locally.

There is no public RFC defining a protocol for port 3516. There is no major software package that uses it by default. If you see traffic on port 3516, it is almost certainly a custom application or something unrelated to smartcards entirely.

What the Registered Range Means

Ports 1024–49151 are registered ports — a middle ground between the well-known ports (0–1023, which require root/administrator privileges to bind) and the ephemeral ports (49152–65535, used temporarily by operating systems for outbound connections).

Anyone can apply to IANA to register a port in this range. The bar is low. The result is a registry full of names like "Smartcard Port" — historically notable, technically valid, practically invisible. ²

How to Check What's Actually on This Port

If port 3516 is open on a machine you control or are inspecting, don't assume it's smartcard-related. Use standard tools to find out what's actually listening:

On Linux/macOS:

# Show which process is listening on port 3516
ss -tlnp | grep 3516

# Or with lsof:
lsof -i :3516

On Windows:

# Show listening processes with PIDs
netstat -ano | findstr :3516

# Match the PID to a process
Get-Process -Id <PID>

If something is listening and you don't recognize it, treat it as worth investigating — an open port with unknown ownership is always a question worth answering.