Port 645 sits in a strange place in the Internet's infrastructure. It's officially assigned. It has a name in the IANA registry. But if you ask what actually uses it, the answer gets complicated.
The Official Assignment
According to IANA, port 645 is assigned to PSSC (both TCP and UDP).1 The contact listed is Egon Meier-Engelen. That's all the official documentation tells us.
What is PSSC? That's where the trail goes cold. Unlike protocols with active communities, documentation, and ongoing development, PSSC exists primarily as a registry entry. The protocol itself appears to have faded into obscurity, leaving behind only its port number reservation.
What Actually Uses Port 645
In the absence of a dominant official protocol, port 645 has become a kind of digital commons where various services have set up shop unofficially:
Microsoft DirectPlay — Some sources report that Microsoft's DirectPlay gaming protocol used port 645 for peer-to-peer game connectivity. DirectPlay itself is now deprecated, replaced by Xbox Live and other modern gaming infrastructure.
Novell eDirectory — Port 645 has been associated with Novell Directory Services and eDirectory for network file transfer and synchronization.2
DIS Protocol — The Distributed Interactive Simulation protocol, used in military and defense simulations, has been reported to use port 645 for communication between simulation participants.3
Malware — Like many lesser-used ports, 645 has occasionally been exploited by trojans and viruses for command-and-control communication.4
None of these uses are official IANA assignments. They're just services that found an available port number and used it.
The Well-Known Range
Port 645 falls in the well-known ports range (0-1023), which means it's supposed to be assigned by IANA for specific, standardized services. These ports require root privileges to bind to on Unix-like systems—a security measure that assumes only trusted, official services should be listening here.
But the reality is messier than the design. A well-known port with an obscure official assignment becomes an attractive target for other services looking for a home.
Why This Matters
Port 645 tells a story about the Internet's infrastructure: not every official assignment becomes important. Some protocols get registered, assigned a port number, and then... nothing. They never gain adoption. The company or person behind them moves on. The documentation disappears.
But the port number remains reserved. And in that vacuum, other services move in, creating an unofficial ecosystem that's just as real as the official one—maybe more so, since it's what people actually encounter in the wild.
Checking What's Listening
If you want to see if anything is using port 645 on your system:
On Linux or macOS:
On Windows:
If something is listening, you'll see the process ID and can investigate what service is actually using the port.
Related Ports
- Port 646 — Assigned to LDP (Label Distribution Protocol), actually used
- Port 644 — Unassigned
- Port 389 — LDAP, often used alongside directory services that might use 645
Frequently Asked Questions About Port 645
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