Port 721 sits in the well-known port range (0-1023) but has no official IANA assignment. It belongs to that strange category of ports that once mattered and now don't—a ghost of a specific implementation choice made three decades ago.
What Range This Port Belongs To
Port 721 is in the well-known port range (0-1023), also called the system ports. This range is controlled by IANA and typically reserved for services that need privileged access or standardized assignment.
But port 721 never got that standardized assignment. It exists in the numbering space, but no protocol officially claims it.
The Windows NT 3.5 Printer Story
Here's the weird part: for a brief period in the mid-1990s, port 721 actually mattered.1
Windows NT 3.5 used ports 721-731 as source ports when sending print jobs to printers via LPD (Line Printer Daemon) on port 515. When your computer sent a print job, it didn't use a random high-numbered port like modern systems do. It picked sequentially from this narrow eleven-port range.
Why? Probably because someone at Microsoft in 1995 thought it would be easier to manage, firewall, or debug if print traffic always came from a predictable place.
Then Windows NT 4.0 came out and changed everything. The new version switched to using random source ports between 512-1023, and the 721-731 scheme vanished.2 Port 721 went from being part of a specific implementation to being... nothing.
What This Port Is Used For Today
Nothing official.
Some network monitoring tools and old documentation still reference the Windows NT 3.5 printing behavior, but no modern system uses port 721 for anything standardized. It's unassigned, unclaimed, and unremarkable.
Occasionally, malware has been reported using random ports including 721, but that's true of almost any unassigned port—if no legitimate service is listening, it's available for anything.3
How to Check What's Listening on This Port
If you want to see if anything on your system is using port 721:
On Linux or macOS:
On Windows:
Using nmap to scan a remote host:
Odds are, nothing is listening. Port 721 is almost always closed on modern systems.
Why Unassigned Ports Matter
The existence of unassigned ports like 721 is actually important to how the Internet works.
Not every port needs a service. In fact, most ports are empty most of the time. The 65,535 available port numbers aren't a list of services—they're a namespace. A way for your computer to keep track of multiple simultaneous connections.
When you open a web page, your browser doesn't connect from port 80. It picks a random high-numbered port (usually 49152-65535, the ephemeral range) and connects to port 443 on the server. The server responds to your ephemeral port. That's how you can have fifty browser tabs open at once—each one gets its own source port.
Unassigned well-known ports like 721 exist because the Internet needed a numbering system, not because every number would be used. They're the empty seats in a theater. They're there if you need them, but most of the time, silence is fine.
The Honest Answer
Port 721 is a historical footnote. It mattered for one version of one operating system's printing implementation, and then it stopped mattering.
There's no protocol to learn here. No RFC to read. No service to understand.
Just a port number that once briefly served a purpose, and now serves as a reminder that the Internet is full of decisions that made sense at the time and then got quietly abandoned.
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