Port 3431: NDL Active License Server

What Port 3431 Is

Port 3431 is registered with IANA under the service name ndl-als — the Active License Server for NDL Software Limited, a UK company specializing in public sector digital automation. Think robotic process automation, electronic forms, and workflow tools for government agencies.

The "ALS" handles license verification for NDL's products: when software on a client machine needs to confirm it has a valid license, it reaches out to this server. Port 3431 is where that conversation happens.

The port was registered by Quentin Brown — who LinkedIn now lists as "Retired - Ex NDL Software." He filed the IANA registration in 2002, and it has held its place in the registry ever since. ¹

The Registered Port Range

Port 3431 sits in the registered ports range (1024–49151). These ports are not reserved for system services the way well-known ports (0–1023) are — you don't need elevated privileges to bind to them. Instead, they're registered by organizations with IANA to signal: "we intend to use this port for this purpose."

Registration doesn't enforce anything. Any process on any machine can listen on port 3431. The registry is a coordination system, not an access control system. It just means that if you see traffic on 3431, there's an official explanation for what should be there.

What You'll Actually Find on Port 3431

Unless you're inside an organization running NDL Software products, you won't see legitimate traffic here. In the wild, unexpected activity on 3431 is more likely to be:

• A misconfigured application picking an arbitrary port
• Malware using an obscure registered port to blend in
• A custom internal service that chose this port informally

The IANA registration gives you a baseline: if 3431 is open on a machine that has no NDL Software products, it's worth asking why.

How to Check What's Listening

On Linux or macOS:

ss -tlnp | grep 3431
# or
lsof -i :3431

On Windows:

netstat -ano | findstr :3431
# Then look up the PID:
tasklist | findstr <PID>

If something is listening and it's not NDL Software's license server, you have your answer.

Why Obscure Registered Ports Matter

The registered port range contains thousands of entries like this one — services from companies large and small, some still active, some long since abandoned, some registered by people who have retired and moved on. They're the Internet's equivalent of reserved parking spots in a lot where most of the cars never show up.

They matter because they're part of how the Internet coordinates without central enforcement. When two different pieces of software both want to use port 3431, someone has to yield. The registry is the record of who got there first.

Port 3431 belongs to NDL Software. That's the whole story.