Port 791 has no officially assigned service. It exists in the well-known port range—the first 1,024 ports reserved for system-level services—but nobody has claimed it.
What the Well-Known Range Means
Ports 0-1023 are called "well-known ports" or "system ports." The Internet Assigned Numbers Authority (IANA) controls this range. You can't just start using one of these ports. You need to apply to IANA, explain why your protocol needs a well-known port number, and wait for approval.1
The well-known range exists because some services need to be findable at predictable addresses. When your browser connects to a web server, it knows to try port 80 for HTTP and port 443 for HTTPS. When your email client fetches mail, it knows port 143 is IMAP and port 993 is IMAP over TLS. These numbers work because everyone agrees on them.
Port 791 sits in this privileged range but serves no protocol. It's reserved space—held in case someone needs it.
Why Unassigned Ports Matter
The well-known port range is surprisingly sparse. Only about 300 of the 1,024 well-known ports have official assignments. The rest, like 791, remain unassigned.
This isn't waste—it's preparation. IANA keeps these ports available for future protocols. When someone invents a new critical Internet service that needs a predictable port number, there needs to be room for it.
Unassigned ports also prevent conflicts. If port 791 were free-for-all, different applications might use it for incompatible purposes. Your firewall wouldn't know what to allow. Network monitoring tools wouldn't know what they're seeing. The registry prevents this chaos by keeping unassigned ports explicitly unassigned until someone requests them properly.
Checking What's Listening
Even though port 791 has no official assignment, something might be listening on it on your system. Applications can bind to any port the operating system allows—official assignment is about standards, not technical restrictions.
On Linux or macOS, check what's using port 791:
On Linux, you can also use:
On Windows:
If you find something listening on port 791, it's either a local application that chose this port arbitrarily, or someone using an unofficial protocol that never got registered with IANA.
Related Ports
Other unassigned well-known ports in the same neighborhood:
- Port 788 - Unassigned
- Port 789 - Unassigned
- Port 790 - Unassigned
- Port 792 - Unassigned
- Port 793 - Unassigned
Nearby assigned ports:
- Port 787 - QSC (Quality of Service Control)
- Port 800 - mdbs_daemon (Medical Device Broker Service)
Frequently Asked Questions About Port 791
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