What This Port Is
Port 1004 is a well-known port (0–1023)—the range officially assigned by the Internet Assigned Numbers Authority (IANA). These ports are supposed to be reserved for widely recognized services: HTTP at 80, HTTPS at 443, DNS at 53. Port 1004 has no such assignment. It exists in the registry, documented and catalogued, but unclaimed. 1
The Well-Known Port Range
The 0–1023 range represents the Internet's reserved space—ports that require special permissions to use on Unix systems and are supposed to host only officially registered services. IANA maintains a comprehensive registry of these assignments. 2 Most of this range is accounted for: old protocols like Telnet (23), SMTP (25), FTP (20–21). But gaps exist. Port 1004 is one of them.
No Unofficial Uses
Unlike some unassigned ports that get claimed by rogue services or internal tools, port 1004 has no known widespread unofficial use. 1 It doesn't host private services on major platforms. No major software defaults to it. It simply sits unused—a curiosity without an application.
How to Check What's Listening
If you suspect something is running on port 1004 on your system:
On macOS or Linux:
On Windows:
From another computer (if you have access):
The port will almost certainly be silent. That silence is the honest answer.
Why Unassigned Ports Matter
The well-known port range is finite. IANA is judicious about assignments—each port carries weight, a promise to the Internet that this number maps to a specific service. Unassigned ports like 1004 represent restraint: the Internet could assign every port, but it doesn't. It reserves space for future protocols, for services not yet invented, for problems not yet understood. 2
Port 1004 is a reminder that the Internet doesn't need to fill every space. Some doors are left closed, waiting for the right key.
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