Port 673 sits in the well-known ports range (0-1023), officially assigned by IANA to a protocol called CIMPLEX. If you search for what CIMPLEX actually does, you'll find almost nothing. The protocol exists in port registries but not in practice. Documentation has vanished. It's a ghost in the registry.
What Lives Here (Or Used To)
Official assignment: CIMPLEX (both TCP and UDP) Reality: Essentially unused
The only documented real-world use of port 673 was on older Mac OS X systems (before 2007), where it served NetInfo—Apple's hierarchical distributed database for system administration.12
NetInfo stored user accounts, group configurations, email settings, NFS mounts, printer configurations, and other administrative data. It was part of Mac OS X's RPC-based services, inherited from NeXTSTEP.3
Then Mac OS X Leopard shipped in October 2007, and NetInfo disappeared. Port 673 went quiet.
The Well-Known Range
Ports 0-1023 are called "well-known ports" or "system ports." They're assigned by IANA through formal procedures—either IETF Review or IESG Approval. Getting a port in this range requires documentation, a clear purpose, and approval from the Internet Engineering Task Force.
CIMPLEX got one. Then it disappeared from use.
This happens more than you'd think. The well-known range is full of ports assigned to protocols that never caught on, or that solved problems that no longer exist, or that were replaced by something better. Port 673 is one of them.
Why Unassigned (Or Forgotten) Ports Matter
The port numbering system has 65,535 ports. Most of them do nothing most of the time. That's fine. That's how it works.
But the well-known range is supposed to be different. These are the ports that matter—the ones where you can expect to find DNS (53), HTTP (80), HTTPS (443), SSH (22). The foundation of the Internet lives here.
When a well-known port goes unused, it represents a missed opportunity. That number could have been assigned to something that actually needed it. But the registry is conservative by design—once assigned, ports rarely get reclaimed. CIMPLEX still owns 673, even though nobody uses it.
The result: we have a well-known port range cluttered with forgotten protocols, while new services fight for assignments in the higher ranges.
Checking What's Listening
If you want to see what's actually using port 673 on your system:
On Linux or macOS:
On Windows:
Most likely, you'll find nothing. Port 673 is assigned but silent—a reminder that the Internet's infrastructure contains layers of abandoned history, protocols that mattered once and then didn't, ports that carry nothing but the memory of what used to flow through them.
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