Port 528 sits in the well-known range with an official assignment, but if you're looking for active traffic here, you're probably looking in the wrong place.
What Is Port 528?
Port 528 is officially assigned to Customer IXChange (service name: custix), registered with IANA for both TCP and UDP protocols.1 The registration was last updated in January 2022 with Ferdi Ladeira listed as the assignee.
That's essentially all we know. The protocol itself has left almost no documentation trail, no RFCs, no widespread implementation, and no community of users discussing it online.
The Well-Known Range
Port 528 falls within the well-known ports range (0-1023), also called System Ports. These ports are assigned by IANA through formal review processes and are supposed to be reserved for services important enough to deserve universal recognition.2
The well-known range is valuable real estate. Getting a port here means IANA believed your service mattered. But assignment doesn't guarantee use. The Internet is littered with protocols that got official port numbers and then faded into obscurity.
Why This Port Matters (Even Though It's Silent)
Port 528 represents something honest about how the Internet works: not every plan succeeds.
Someone designed Customer IXChange. Someone submitted the paperwork to IANA. Someone thought this protocol would be important enough to deserve a well-known port. And then... nothing. Or almost nothing. Maybe the service ran somewhere for a while. Maybe it solved a problem that later became irrelevant. Maybe it was replaced by something better.
The port remains assigned. The door is still there. But nobody's knocking.
Checking What's Actually on Port 528
Even though custix has vanished, port 528 can still be used—by anything. On your machine, you can check what's actually listening:
On Linux/Mac:
On Windows:
You'll probably find nothing. But if something is listening on port 528, it's almost certainly not Customer IXChange. It might be:
- A custom application that chose an unused port
- Testing or development software
- Malware (security scans have occasionally flagged suspicious activity on unused well-known ports)
The Ghost Protocols
The Internet has thousands of these: officially assigned ports for protocols that never gained traction. They're not failures—they're experiments. The Internet grew because people tried things. Most attempts don't survive. That's fine. That's how progress works.
Port 528 is a reminder that even in the carefully curated well-known range, not every door leads somewhere active. The port exists. The assignment is real. But the protocol is a ghost.
Frequently Asked Questions About Port 528
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