Port 1171: AT+C FmiApplicationServer — The forgotten registration

Port 1171 is officially registered with IANA for a service called "atc-appserver" (AT+C FmiApplicationServer), assigned for both TCP and UDP protocols in February 2004.¹²

Here's the strange part: almost no one knows what it does.

The Ghost in the Registry

The Internet Assigned Numbers Authority maintains a registry of over 50,000 port assignments. Most well-known ports have extensive documentation—RFCs, tutorials, Stack Overflow questions. Port 1171 has almost none of that.

What we know:

• Official name: atc-appserver
• Full name: AT+C FmiApplicationServer
• Registration date: February 2004
• Protocol support: TCP and UDP
• Documentation: Essentially nonexistent

This port belongs to the registered ports range (1024-49151), where organizations can request specific port assignments from IANA for their applications. Someone at AT+C requested this port, IANA granted it, and then... silence.

Why Obscure Ports Matter

Port 1171 represents something important about how the Internet actually works: not every port carries billions of packets. Not every registration becomes a standard. Some ports are assigned for enterprise applications that only ever run inside specific organizations. Some are for products that never launched. Some are for services that came and went before the Internet learned to remember everything.

The registry doesn't judge. It just records. Port 1171 exists in the official record alongside HTTP and SSH, even though you'll probably never see traffic on it.

What This Means for You

If you see port 1171 active on your network, it could be:

1. The actual AT+C FmiApplicationServer (unlikely but possible)
2. A different application using the port unofficially
3. Malware exploiting an obscure port number
4. A misconfiguration or port collision

Checking What's Listening

To see if anything is using port 1171 on your system:

Linux/macOS:

sudo lsof -i :1171
# or
sudo netstat -tulpn | grep 1171

Windows:

netstat -ano | findstr :1171

If something is listening, the output will show you which process is using it.

The Registered Ports Range

Port 1171 sits in the middle of the registered ports range (1024-49151). These ports require IANA registration but don't have the strict standardization of well-known ports (0-1023). Organizations can request specific ports for their applications, which is how AT+C secured 1171 for their FmiApplicationServer.

Unlike well-known ports, registered ports often serve niche purposes. They're assigned to specific companies or projects, and many see little to no traffic outside their intended environments.