Port 3645: Cyc — The Port That Tried to Know Everything

What This Port Is

Port 3645 (TCP and UDP) is officially registered with IANA under the service name Cyc. The registration was filed in January 2003 by Stephen Reed of Cycorp, Inc.¹

It is not unassigned. It belongs to a ghost.

What Cyc Was

Cyc was the most ambitious AI project ever attempted.

In July 1984, Douglas Lenat began an experiment at the Microelectronics and Computer Technology Corporation in Austin, Texas. The premise: general AI had been failing because machines lacked common sense — all the implicit knowledge humans carry without thinking about it. You know that broken glass can cut you. That people sleep at night. That if someone is taller than a doorframe, they'll have to duck. A machine trained on text doesn't know any of this unless it's been told explicitly.

Lenat's solution was brute force. Hire a team of knowledge engineers. Have them read encyclopedias, books, articles. Manually encode every fact, rule, and relationship. Do this for decades until the machine simply knows what humans know.²

The result was called the Cyc Knowledge Base, built in a custom language called CycL. By the time the project ended, it contained roughly 30 million assertions, representing 239,000 concepts — things like PhysicalObject, TemporalThing, Agent, and millions of relationships between them.³

It cost approximately $200 million and consumed an estimated 2,000 person-years of effort.

The Port and the Server

Cycorp registered port 3645 for the Cyc inference engine's network interface — the socket through which client applications could query the knowledge base.⁴ When OpenCyc (the open-source version of Cyc) was released in 2002, the server would listen on this port and respond to queries written in CycL or SubL, Cyc's inference language.

So port 3645 was, for a time, the door into a machine that was supposed to understand the world.

What Happened

OpenCyc 4.0 shipped in June 2012. It contained 239,000 concepts and over 2 million assertions. Cycorp was proud of this. They should have been — it represented decades of painstaking work.

But the world had moved on. Large language models were beginning to demonstrate that statistical pattern matching over massive text corpora could produce surprisingly human-like reasoning — without the expensive, brittle, hand-encoded knowledge graphs. The approach that Lenat had built his career opposing was winning.

OpenCyc went offline in March 2017. Cycorp cited "confusion amongst users" caused by fragmentation of the open-source community.⁵

Douglas Lenat spent his final years arguing that Cyc and large language models were complementary — that LLMs needed Cyc's structured reasoning, and Cyc needed LLMs' flexibility. He wrote a paper about it. He died in August 2023, still believing.

Port 3645 remains registered in the IANA database. Nothing actively listens on it.

Checking What's on This Port

If you see traffic on port 3645 on a modern network, it isn't Cyc. It's either:

• A misconfigured or repurposed application using the port opportunistically
• A port scanner cataloging your open ports
• Malicious software that happens to use this range

To check what's listening on your system:

Linux/macOS:

# Show processes listening on port 3645
ss -tlnp sport = :3645

# Alternative with lsof
lsof -i :3645

Windows:

# Show what's bound to port 3645
netstat -ano | findstr :3645

Why This Port Matters

Port 3645 is a minor curiosity in the IANA registry. But it's also a small monument.

The registered ports range (1024-49151) is full of projects that seemed important when they claimed their number. Many are now defunct, their protocols abandoned, their servers long ago powered down. Port 3645 is one of those — a 20-year-old reservation for a 40-year-old dream.

The Internet's port space is partly a history of what humans thought computing would become. Sometimes those bets paid off. Sometimes a port just sits there, registered to a ghost, long after the machine it pointed to went quiet.

Quick Facts