Port 675 sits in the IANA registry with a single word next to it: DCTP. Sometimes referenced as "Data Center Transfer Protocol" or "Delivery Control Transfer Protocol," depending on which old database you're reading. But the truth is simpler and stranger: nobody knows what DCTP actually does.
There's no RFC. No specification. No implementation anyone can find. Just a port number, assigned decades ago, waiting for a protocol that never arrived.
What Lives in the Registry
According to the IANA Service Name and Transport Protocol Port Number Registry, port 675 is assigned to DCTP for both TCP and UDP.1 That's the extent of the official documentation.
The assignment exists. The protocol doesn't.
The Well-Known Range
Port 675 falls in the well-known ports range (0-1023), which makes its obscurity even more notable. These ports were supposed to be reserved for fundamental Internet services—the protocols that would define how the network operates.
Being in this range means someone, at some point, believed DCTP would be important. Important enough to claim one of the limited spots in the range that includes HTTP (80), HTTPS (443), SSH (22), and DNS (53).
They were wrong. Or they were early. Or the project died before it shipped.
Why Port 675 Matters
It doesn't—at least not in the way its creators intended. But it matters as a reminder of something true about the Internet: not every door gets opened.
The port number system was designed when addresses felt infinite and the future felt certain. If you were building something that needed network communication, you could request a port number and IANA would assign you one. No proof of concept required. No adoption threshold to meet.
Port 675 is what happens when that optimism meets reality. Someone had an idea. They registered a port. The idea didn't survive contact with the world.
Checking What's Actually There
Even though DCTP isn't a real service, port 675 could theoretically be used by something on your system. To check:
On Linux/macOS:
On Windows:
If something appears, it's not DCTP. It's something else that decided to use an empty slot.
The Ghost Ports
Port 675 isn't alone. The IANA registry is full of assignments like this—services that were registered but never deployed, protocols that existed only in draft documents, ideas that died in committee.
They're not mistakes. They're artifacts. Evidence that the Internet was built by people trying things, most of which didn't work.
Port 675 is assigned to a protocol that never existed. And it will stay that way, forever, because port numbers are never reclaimed. The registry doesn't forget.
Related Ports
- Port 674 - ACAP (Application Configuration Access Protocol)
- Port 676 - VPPS-VIA (another obscure assignment)
- Port 677 - Virtual Presence Protocol
Frequently Asked Questions About Port 675
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