What Port 425 Was For
Port 425 is officially registered to ICAD-EL (ICAD Emacs Lisp), an interface protocol for ICAD — Intelligent Computer-Aided Design.1 ICAD was a knowledge-based engineering system that ran on Symbolics Lisp Machines during the 1980s and 1990s, during the height of the AI boom.2
The protocol allowed Emacs Lisp to communicate with the ICAD system for constraint-based computer-aided design work. Engineers could encode design knowledge using semantic representations, and the system would automate routine design tasks through rule-based expert systems.3
The Hardware That No Longer Exists
ICAD ran on specialized Lisp Machines built by Symbolics — dedicated computers optimized for running Lisp code and AI applications.4 These machines cost tens of thousands of dollars and were marketed specifically for AI research and development during the 1980s, when companies like DARPA were pouring money into symbolic AI.
The Symbolics Lisp Machine was supposed to be the future. ICAD was its most popular application.4 Then the AI Winter came.
By the late 1980s and early 1990s, the AI boom collapsed. DARPA cut funding. Symbolics struggled and eventually filed for bankruptcy. The Lisp Machines became museum pieces.2 ICAD was eventually ported to Unix systems running Common Lisp, but the original hardware-specific protocol on port 425 became obsolete.
What Happened to This Port
Port 425 remains in the IANA registry, registered to Larry Stone for ICAD-EL.1 But the service it was designed for no longer exists in its original form. The Symbolics machines are gone. The company is defunct. ICAD itself was acquired and evolved into modern knowledge-based engineering tools, but not using port 425.
This is what happens to ports when the technology they served disappears. The number stays registered — a tombstone in the Internet's address space.
Why Obsolete Ports Matter
Port 425 teaches something important about the well-known port range (0-1023): these numbers were assigned when the Internet was young, when people believed certain technologies would be permanent. Some were right. Port 22 for SSH is still essential. Port 80 for HTTP changed the world.
But some were wrong. Port 425 for ICAD-EL serves hardware that no longer exists, running software that was redesigned decades ago. The port number remains because removing assignments is complicated and rarely necessary — the Internet is not running out of port numbers.
These ghost ports are historical markers. They show you what people thought would matter. What they bet on. What died when the funding dried up.
Well-Known Port Range
Port 425 sits in the well-known port range (0-1023), which requires root/administrator privileges to bind to on Unix-like systems. These ports were originally assigned by IANA for specific services that were expected to be permanent and universal.
In practice, many well-known ports are rarely used. Some, like port 425, are effectively abandoned. Others have been repurposed unofficially. The well-known range is a snapshot of what the Internet's architects thought would be important in the 1980s and early 1990s.
Security Note
Port 425 has occasionally been flagged in security databases as being exploited by trojans or malware.5 This is common for obscure, unused ports — attackers sometimes use them precisely because they're not expected to have legitimate traffic. If you see unexpected traffic on port 425, investigate it.
How to Check What's Listening
On Linux or macOS:
On Windows:
If nothing is listening, that's normal. This port is a relic.
The Genuine Strangeness
Port 425 is a ghost from the AI boom. It was registered for a technology that was supposed to revolutionize engineering with artificial intelligence — expert systems that could encode and reason about design knowledge, running on dedicated hardware optimized for symbolic processing.
Then the AI Winter came. The hardware disappeared. The company went bankrupt. The technology evolved into something that didn't need this port anymore.
But the port number remains, sitting in the IANA registry, waiting for connections that will never come from machines that no longer exist.
This is the strange immortality of port numbers. The Symbolics Lisp Machines are museum pieces now. You can't buy one. You can't run ICAD-EL on the original hardware. But port 425 is still there, reserved, a permanent record of what people thought would be permanent.
Every obsolete port is a time capsule. Port 425 is from the era when people believed symbolic AI and Lisp Machines were the future. They were wrong about that, but the port number remembers what they believed.
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