Port 840 is unassigned. It has no official service, no RFC defining its purpose, no protocol waiting for connections.
What This Means
Port 840 sits in the well-known port range (0-1023)—the space reserved for fundamental Internet services. These ports are controlled by the Internet Assigned Numbers Authority (IANA), which carefully assigns them to protocols that need a stable, predictable address.
But port 840 was never assigned. It's part of a block (834-846) that remains empty in the official registry.1
Why Unassigned Ports Exist
Not every port number has a purpose. The well-known range has 1,024 slots, and only the protocols that genuinely need a reserved address get one. Port 840 is simply unclaimed territory.
This doesn't mean nothing could run there. Any application can listen on port 840 if it wants to. But without an official assignment, there's no standard service you'd expect to find.
How to Check This Port
If you want to see if anything is listening on port 840 on your machine:
On Linux or macOS:
On Windows:
If something appears, it's a custom application—not a standard Internet service.
The Bigger Picture
The port system works because of assignments like port 22 for SSH or port 443 for HTTPS. Those numbers mean something consistent everywhere. Port 840 is the opposite: it means nothing in particular, anywhere.
And that's fine. The Internet doesn't need all 65,535 ports to have official purposes. Some ports exist as empty space, available if a future protocol needs them. Port 840 is one of those—reserved, but waiting for a purpose that might never come.
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