What Range This Port Belongs To
Port 1740 sits in the registered ports range (1024-49151). IANA maintains this range as a registry where vendors can formally claim a port for their software. Port 1740 has no such claim — it's listed as unassigned.
Unassigned doesn't mean unused. It means no one has filed paperwork.
Observed Unofficial Uses
CODESYS Industrial Automation (UDP 1740-1743)
The most documented use of port 1740 is by CODESYS, a widely used development environment for programming industrial controllers (PLCs). CODESYS Runtime listens on UDP port 1740 and sends broadcast packets across the local network so that the development software can discover which PLCs are reachable.
The full range it occupies:
| Port | Protocol | Role |
|---|---|---|
| 1740 | UDP | CODESYS device discovery broadcast |
| 1741 | UDP | CODESYS communication |
| 1742 | UDP | CODESYS communication |
| 1743 | UDP | CODESYS communication |
The TCP equivalents shift to 11740-11743.
Schneider Electric's M241 and M251 PLCs use this range for their Machine Expert Protocol. If you run Wireshark on a network with Schneider hardware and see a steady stream of UDP broadcasts landing on 1740, that's your PLC announcing itself.
These broadcasts are local-only — they don't cross routers — so the chattiness stays contained to the factory floor segment.
How to Check What's Listening on Port 1740
On Linux or macOS:
On Windows:
With Wireshark:
Filter for traffic on this port to see whether it's UDP broadcasts (CODESYS discovery) or something else entirely:
Why Unassigned Ports Matter
The port registry exists so software can find each other reliably. When a vendor skips registration and uses an unassigned port anyway, it works — until it doesn't. Two applications on the same machine might both reach for 1740, and whoever starts first wins.
CODESYS's use of 1740-1743 is stable in practice because industrial networks are controlled environments. The PLCs don't share machines with general-purpose software. But on a mixed network, the lack of a formal claim means there's no guarantee.
If you're designing software that needs a port, register it. The process is free. The chaos it prevents is not.
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