Port 666 was officially registered to id Software for the original Doom game's multiplayer mode in 1993.1 It remains one of the very few video games to ever receive an official well-known port assignment from IANA.
The number choice was, of course, deliberate. Doom is a game about fighting demons. Port 666. The joke wrote itself.
What Range This Port Belongs To
Port 666 sits in the well-known ports range (0-1023)—the most restricted tier of the port system, typically reserved for fundamental Internet services like HTTP, DNS, and email.2 Receiving assignment in this range requires IANA approval.
The fact that a video game secured a well-known port is genuinely unusual. It reflects both the cultural significance of Doom in the early Internet era and id Software's foresight in formally registering their port rather than just picking a random number.
How Doom Used Port 666
Doom's multiplayer mode used UDP port 666 for networked gameplay. In the early 1990s, before matchmaking services and game lobbies, players needed to know specific IP addresses and port numbers to connect to each other. Port 666 became Doom's default meeting point.
The protocol was simple by modern standards—packet-based position updates, weapon fire, and game state synchronization. But it worked. And it introduced millions of people to networked gaming for the first time.
The Security Problem
Port 666's fame became its curse. The number's connotations and the port's recognition made it an attractive target for malware authors. Over the years, port 666 has been used by numerous trojans and backdoors precisely because network administrators would notice it.1
The attackers weren't trying to hide. They were making a statement. Using port 666 was deliberate provocation—a digital calling card.
This means that if you see port 666 open on a modern system that isn't running vintage Doom, you should investigate. Legitimate uses are rare.
The Windows Legacy
Windows has reserved port 666 for Doom since Windows 95, and the reservation still exists in Windows 10 and 11.3 You can see it in the services file on any Windows machine. It's a 30-year-old easter egg that became permanent infrastructure.
The reservation doesn't mean Windows blocks the port or that anything listens on it by default. It just means the number is officially spoken for. A small acknowledgment that Doom was here.
How to Check What's Listening
On Linux or macOS:
On Windows:
If you see something listening on port 666 that isn't deliberately running Doom, investigate immediately.
Why This Port Matters
Port 666 is a reminder that the Internet was built by humans with a sense of humor. Someone at IANA could have said no. They could have told id Software to pick a number in the registered range (1024-49151) like everyone else.
But they didn't. They gave Hell's port number to a game about fighting demons, and it stuck.
The port is rarely used now—modern games use dynamic port allocation and matchmaking services. But it's still there in the IANA registry, still reserved in Windows, still carrying the ghost of 1993 when networked Doom was the future and port 666 was the address of that future.
Frequently Asked Questions About Port 666
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