Port 640 sits in the well-known port range (0-1023), officially registered with IANA1 for a service called "entrust-sps"—Entrust Security Provider Service. Both TCP and UDP. The registration is real. What's less clear is whether anything still uses it.
What Entrust SPS Was
Entrust built enterprise security infrastructure in the 1990s and 2000s—PKI systems, certificate management, authentication services. The Entrust Security Provider was middleware that handled cryptographic operations: smart card authentication, digital signatures, encrypted email, VPN connections.2
Port 640 was assigned to some component of this system. The exact protocol, the specific service that listened here—that's harder to find. The documentation has aged out. The service has been replaced by newer systems. What remains is the port number in the registry, a marker of something that once ran on enterprise networks.
The Well-Known Range
Ports 0-1023 are the well-known ports, assigned by IANA for standardized services. Getting a well-known port meant your protocol mattered—or was expected to matter. HTTP got 80. HTTPS got 443. SMTP got 25. And Entrust got 640.
Not every well-known port became essential infrastructure. Some, like port 640, were assigned when a service seemed important, when enterprise PKI was the future of security. The registration persists even after the service fades.
Why Unassigned or Forgotten Ports Matter
The port registry is a historical record. Port 640 tells you that in the late 1990s, someone at Entrust filed paperwork with IANA, made a case that their security service deserved a well-known port, and got approval. That service is now obscure enough that finding documentation requires archaeology.
This happens. Protocols die. Companies pivot. Services get replaced. But port numbers are forever—or at least, they're rarely reclaimed. The registry grows, accumulates, becomes a museum of networking history.
If you scan port 640 today and find something listening, it's probably not Entrust SPS. It might be:
- A legacy system that never got updated
- Something else repurposing an obscure port
- Nothing—most ports sit idle most of the time
How to Check What's Listening
On Linux or macOS:
On Windows:
If nothing returns, nothing's listening. That's the most likely outcome for port 640 in 2026.
What This Port Carries
Once: enterprise authentication traffic, certificate requests, cryptographic operations for companies running Entrust infrastructure.
Now: probably nothing. Possibly legacy systems in organizations that deployed Entrust in the early 2000s and never migrated.
Port 640 is a reminder that the well-known port range isn't just active services—it's also ghosts. Registrations that outlived the protocols they represented. Digital markers of technologies that mattered, briefly, and then didn't.
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