Port 312 sits in the well-known ports range with an official assignment, but it's assigned to something that barely exists. VSLMP—Very Simple Local Mailer Protocol—has a name, a port number, and a registrant. What it doesn't have is a history, an RFC, or anyone who remembers using it.
What Port 312 Is Assigned To
According to the IANA registry, port 312 is officially assigned to VSLMP for both TCP and UDP protocols.1 The acronym stands for "Very Simple Local Mailer Protocol."2
That's where the story ends. Or rather, where it never begins.
The Protocol That Isn't There
No RFC documents VSLMP. No technical specifications explain how it works. No historical accounts describe its creation or use. The protocol appears in acronym databases with its full name, but that's the extent of its footprint on the Internet.
The contact listed in the IANA registry is Gerben Wierda, who became known for enterprise architecture work and the ArchiMate modeling language—not email protocols.3 Whether VSLMP was an experimental protocol, an internal tool that never saw wider use, or something else entirely remains unclear.
The Well-Known Ports Range
Port 312 falls within the well-known ports range (0-1023), which IANA reserves for system-level services that provide fundamental network functionality. Ports in this range typically require superuser privileges to bind on Unix-like systems.
Being assigned a well-known port usually signals importance—these are the ports that carry DNS, HTTP, SMTP, SSH, and other protocols that form the Internet's foundation. But port 312 sits among these giants carrying nothing anyone can identify.
What This Means
Most ports in the well-known range either:
- Carry active protocols with clear documentation (like port 25 for SMTP or port 22 for SSH)
- Are marked as "Unassigned" in the registry
- Have historical significance even if no longer widely used
Port 312 is different. It has an assignment but no presence. It's registered but not used. It exists in the official record but not in practice.
Why Unassigned and Obscure Ports Matter
The IANA port registry is more than a phone book for protocols. It's an archaeological record of the Internet's development—what people tried to build, what succeeded, what failed, and what was abandoned.
Ports like 312 are reminders that not every protocol succeeded. Not every assignment led to adoption. Not every registered service became part of the infrastructure we depend on. Some protocols were experiments that never escaped the lab. Some were internal tools that never needed to be public. Some were ideas that simply didn't work out.
The registry preserves these ghosts alongside the giants, maintaining a complete record even when the reason for an assignment has been lost to time.
How to Check What's Listening
If you want to see whether anything is actually using port 312 on your system:
On Linux or macOS:
On Windows:
Most likely, you'll find nothing. Port 312 carries its official assignment but remains silent, waiting for a protocol that may never truly have existed.
The Mystery Remains
Port 312 is a peculiarity in the registry—officially assigned but functionally absent. Whether VSLMP was a forgotten experiment, a protocol that never launched, or something lost to time, we may never know.
What remains is the placeholder: a port number, a protocol name, and the absence of everything else that usually makes a protocol real.
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