Port 739: Unassigned — A numbered door with nothing behind it

Port 739 exists in the well-known port range but has no service assigned to it. According to IANA, ports 732-740 are currently unassigned for both TCP and UDP.¹

What Range This Belongs To

Port 739 falls within the well-known ports (0-1023), also called system ports. This range is controlled by IANA and requires IETF Review or IESG Approval for assignments.²

Being in this range means:

• Only IANA can officially assign it to a service
• It's reserved for potential future use
• Applications shouldn't use it without proper registration
• On Unix-like systems, binding to it typically requires root privileges

Why Unassigned Ports Matter

Not every port number has a service. The Internet's port system includes 65,536 possible ports per protocol (TCP and UDP). Many remain unassigned for good reasons:

Room for growth — The Internet evolves. New protocols emerge. Unassigned ports provide space for future services without having to reclaim already-used addresses.

Deliberate reservation — Some ranges stay unassigned to prevent conflicts or maintain organizational structure in the numbering system.

Historical artifacts — Some ports were once assigned to services that became obsolete. Rather than immediately reassigning them, IANA may leave them unassigned to avoid confusion.

What Could Be Listening Here

Just because IANA hasn't assigned port 739 doesn't mean nothing could be using it. Applications can use any port number, and unassigned ports sometimes see informal use:

• Custom internal services
• Development and testing environments
• Malware (which often exploits unassigned ports precisely because they're not monitored)
• Misconfigured applications

How to Check What's Using Port 739

On Linux or macOS:

# See what's listening on port 739
sudo lsof -i :739

# Or using netstat
sudo netstat -tulpn | grep :739

# Or using ss
sudo ss -tulpn | grep :739

On Windows:

# See what's using port 739
netstat -ano | findstr :739

If something is listening on port 739 on your system, you should verify what it is—unassigned ports being used unexpectedly can indicate either a legitimate custom application or a security concern.

The Structure of Emptiness

Port 739 is one of thousands of unassigned addresses in the port system. These gaps aren't mistakes—they're breathing room. The Internet was built with space to grow, and unassigned ports represent that intentional flexibility.

The IANA port registry isn't just a list of what exists. It's also a map of what's possible. Port 739 waits, numbered and reserved, for whatever comes next.