Port 511 sits in the well-known ports range (0-1023), the section of the Internet's addressing system reserved for standardized services assigned by IANA. These are the ports that matter: HTTP on 80, HTTPS on 443, SSH on 22. The ones the Internet depends on.
And then there's port 511.
What PassGo Was
Port 511 was officially assigned to PassGo, a network protocol associated with PassGo Technologies Ltd., a UK-based company that built access and identity management software.12 The company was founded in 2001 and developed products for web access management, privilege management, and one-time password authentication.3
In December 2007, Quest Software acquired PassGo Technologies.4 John Rainford, who registered port 511 with IANA, was PassGo's CEO and became vice president of PassGo products at Quest after the acquisition.5
PassGo Technologies Limited was dissolved on November 30, 2021.6
The port assignment remains.
What This Port Tells Us
Port 511 is a monument to impermanence. IANA assigns port numbers as permanent allocations—the registry is designed to be stable, authoritative, and lasting. But the services behind those ports are built by companies, and companies don't last forever.
PassGo existed for exactly 20 years. Someone at that company—likely Rainford himself—filled out the IANA application form, explained what the protocol did, and requested a well-known port number. IANA approved it. The number was assigned. Port 511 became PassGo's door into the Internet's architecture.
Now the company is gone, the software is gone, and the protocol is extinct. But the port number remains in the registry, a permanent record of something that is no longer used.
The Well-Known Ports Range
Ports 0-1023 are the well-known ports, assigned by IANA through a formal process defined in RFC 6335.7 Getting a well-known port assignment requires IESG approval or IETF review—it's reserved for protocols that matter, protocols that serve a genuine Internet-wide purpose.
Not every application gets a well-known port. Most services use registered ports (1024-49151) or ephemeral ports (49152-65535). Well-known ports are supposed to be for foundational services.
PassGo got one. And then it disappeared.
How to Check What's Listening
If you want to see whether anything is actually listening on port 511 on your system, you can check:
On Linux or macOS:
On Windows:
Most likely, you'll find nothing. PassGo is gone.
Why Unassigned Ports Matter
Port 511 isn't technically unassigned—it's assigned to a service that no longer exists. This is different from a truly unassigned port, which has never been allocated at all.
The distinction matters because the port registry is meant to prevent conflicts. If two different services both tried to use the same port, the Internet would break in small, unpredictable ways. IANA's registry exists to keep that from happening.
But what happens when a service dies? The port remains assigned, permanently reserved for something that will never listen again. It's a kind of architectural debt—a slot in the addressing system occupied by a ghost.
The IANA registry doesn't have a process for reclaiming ports from defunct services. Once assigned, the number stays assigned. Port 511 will likely remain PassGo's forever, even though PassGo is gone.
The Name in the Registry
Somewhere in IANA's records is John Rainford's name, attached to port 511. He built something that mattered enough to need its own port number. He ran a company, sold it to Quest Software, and watched it eventually dissolve.
The software is gone. The company is gone. But the port number remains, and his name is still in the registry.1
That's what port 511 carries now: memory. A reminder that every port number represents someone's work, someone's purpose, someone's attempt to build something that would last.
Most of them don't.
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