Port 239: Unassigned — A Blank Space in the Well-Known Range

Port Number: 239
Status: Unassigned
Range: Well-Known Ports (0-1023)
Protocols: TCP/UDP (both unassigned)

What Port 239 Is

Port 239 is unassigned. According to IANA's official Service Name and Transport Protocol Port Number Registry, neither TCP nor UDP port 239 has been allocated to any service.¹

This is unusual. Port 239 sits in the well-known ports range (0-1023)—the range that requires IETF review or IESG approval for assignment, the range where SSH (22), HTTP (80), and HTTPS (443) live. These are the numbers reserved for the Internet's most critical services.

But port 239 has no service. No protocol. No purpose.

The Well-Known Ports Range

The well-known ports (0-1023) are also called system ports. They're assigned through a formal process defined in RFC 6335.² These ports require:

• IETF Review for Standards Track RFCs
• IESG Approval for other assignments

The process exists because these port numbers are privileged. On Unix-like systems, binding to a port below 1024 requires root privileges. These ports carry the Internet's infrastructure—DNS, email, web traffic, file transfers.

Port 239 is in this range but carries nothing.

Why Unassigned Ports Matter

Unassigned ports aren't useless. They represent capacity—room for the Internet to grow. As new protocols emerge and old ones fade, IANA assigns, de-assigns, and reserves ports.³

When a port is de-assigned, IANA marks the service name as "Reserved" and includes a comment about its historic usage. Port 239 has no such comment. It appears to have never been assigned in the first place.

This distinguishes it from reserved ports, which are held for special purposes or to extend port ranges in the future. Port 239 is simply available—waiting.

How to Check What's Listening on Port 239

Even though port 239 is officially unassigned, software on your machine could still use it. To check:

On Linux or macOS:

sudo lsof -i :239
netstat -an | grep :239
ss -tuln | grep :239

On Windows:

netstat -an | findstr :239

If nothing returns, port 239 is quiet.

The Confusion with 239.x.x.x

Don't confuse port 239 with the IP address range 239.0.0.0/8.

The IP address range 239.0.0.0 to 239.255.255.255 is part of the IPv4 multicast address space, specifically reserved for administratively scoped multicast.⁴ This range is used for private multicast traffic within organizations—local, non-routable, reusable across different domains.

Port 239 and the 239.x.x.x address range are completely unrelated. One is a transport layer concept (port numbers), the other is a network layer concept (IP addresses).

What This Port Carries

Nothing, officially.

But that absence is meaningful. Port 239 is a reminder that the Internet's address space isn't infinite—it's carefully managed. Every number from 0 to 65535 is either assigned, reserved, or waiting. Port 239 is waiting.

There's a quiet tension in the unassigned well-known ports. They sit in the most valuable real estate of the port number space, yet remain unclaimed. Not because they're unusable, but because no protocol has yet needed them enough to justify the formal assignment process.

Port 239 is capacity. Potential. A door that could open to anything—or remain closed forever.

Related Ports

• Port 0: Reserved, used to request a random ephemeral port
• Ports 1-1023: Well-known ports range, requires IETF/IESG approval for assignment
• Other unassigned well-known ports: Scattered throughout the 0-1023 range