Port 3699: KPN-ICW — A Fossil from the Dial-Up Era

What This Port Is

Port 3699 is assigned to KPN-ICW — Internet Call Waiting — registered with IANA in 2003 by KPN, the Dutch national telecommunications provider. ¹ It operates on both TCP and UDP.

The service sits in the registered port range (1024–49151), which means someone formally claimed it and IANA put it in the registry. Whether anyone still actively uses it is another question entirely.

The Problem It Solved

To understand why this port exists, you have to remember what the Internet felt like in 2003 for most home users: dial-up.

Dial-up worked by calling a number. Your modem dialed, the ISP answered, and the two computers negotiated a connection over the same copper wire that carried your voice calls. The consequence was brutal and simple: while you were online, your phone line was dead. Anyone calling you got a busy signal. You were invisible to the phone network.

Internet Call Waiting was the solution. It let your ISP notify you — over the data connection, through software running on your PC — that someone was trying to call you on your phone line. You could then decide: drop the connection and take the call, or let it go to voicemail.

Port 3699 was where that notification traffic would flow.

Why You Probably Won't See It

Broadband killed the problem. DSL and cable modems don't occupy your phone line, so the phone still works while you're online. Millions of people migrated off dial-up through the mid-2000s, and Internet Call Waiting became a solution to a problem nobody had anymore.

The port registration remains in the IANA registry — port numbers don't expire — but KPN-ICW is effectively a fossil. If you see traffic on port 3699 today, it almost certainly isn't an Internet Call Waiting client.

What's Actually Listening on Port 3699 Today

If you see port 3699 active on a system, the most likely explanation is a custom application that chose this port because it was obscure and uncontested — a common practice. To find out what's actually using it:

On Linux/macOS:

# Show what process is listening on port 3699
sudo lsof -i :3699

# Or with ss
ss -tlnp sport = :3699

On Windows:

# Show listening processes with PID
netstat -ano | findstr :3699

# Then look up the PID
tasklist | findstr <PID>

The Registered Port Range

Port 3699 lives in the registered ports range (1024–49151). This range is where IANA records services that have been formally requested and assigned — as opposed to the well-known ports below 1024 (which require root/admin to bind) or the ephemeral ports above 49151 (which the operating system assigns temporarily for outgoing connections).

Registration doesn't mean enforcement. Anyone can run anything on any port. It just means that when KPN asked IANA to record 3699 as theirs for Internet Call Waiting, IANA did so. The registry is a coordination mechanism, not a lock. ²