Port 1743: Unassigned — CODESYS Industrial Discovery

What Range This Port Belongs To

Port 1743 falls in the registered port range (1024–49151). These ports are managed by IANA, which maintains a registry of assignments — but not every number in this range has been claimed. Port 1743 has not been assigned to any service.¹

Unassigned doesn't mean unused. It means IANA hasn't officially blessed any particular application to own this number. In practice, software uses whatever ports it wants.

Observed Unofficial Uses

CODESYS Industrial Controller Discovery (UDP)

The most consistent real-world use of port 1743 is CODESYS device discovery. CODESYS is a development platform that runs inside programmable logic controllers (PLCs) — the computers that control factory floors, water treatment systems, building automation, and industrial machinery. Vendors including Schneider Electric, Beckhoff, WAGO, and many others ship PLCs with CODESYS inside.

When a CODESYS-based PLC powers on, it begins broadcasting UDP packets on ports 1740 through 1743 once per minute. This is the controller announcing its presence — so that the CODESYS development environment, running on an engineer's workstation, can find it automatically on the local network.²

Port 1743 is the upper end of that sweep. TCP counterparts on ports 11740–11743 handle the actual programming sessions after discovery.³

Security note: These broadcasts are meant for closed industrial networks. CODESYS ports exposed to the Internet are a serious vulnerability. The PIPEDREAM malware toolkit — documented by industrial security researchers in 2022 — specifically targeted CODESYS discovery mechanisms to locate and manipulate PLCs on compromised networks.⁴ Industrial control system (ICS) security guidance is consistent: ports 1740–1743 should never be reachable from outside the local network.

Unitrends Backup Agent (TCP)

Some documentation lists TCP port 1743 as used for command and control messages between Unitrends backup appliances and their agents running on protected machines.⁵ This is an unofficial, application-specific choice — not an IANA assignment.

How to Check What's Listening

On Linux or macOS:

# Show processes listening on port 1743
ss -tulnp | grep 1743

# Or with lsof
lsof -i :1743

On Windows:

netstat -ano | findstr :1743

To watch for UDP broadcasts hitting port 1743:

tcpdump -n udp port 1743

If you're on an industrial network and see periodic UDP traffic from multiple IPs — every 60 seconds or so — that's CODESYS controllers checking in.

Why Unassigned Ports Matter

The registered port range has 48,128 slots. Only a fraction are officially assigned. The rest exist as a kind of commons — available for any application to use, without permission, without guarantee that another application hasn't already claimed the same number on your network.

This creates occasional collisions, and it's why firewall rules based on port numbers alone are unreliable. Port 1743 might be carrying CODESYS discovery on one network and backup agent traffic on another. The port number tells you where to look. What you find there depends on what's installed.