Easy BACnet
Why does my BACnet IP device show offline?
Updated 4 November 2026
“Offline” means the head-end or your tool stopped getting answers from the device. More often than not that is a network or addressing problem rather than dead hardware. The usual causes: you are on a different subnet, the network is dropping scan traffic, an access list or firewall is blocking UDP 47808, two devices share a Device ID or MS/TP MAC, or the device has really lost power or its network link. First make sure the device itself is not answering; a point with a Fault flag on a device that does answer is a different problem.
Offline or fault?
People use “offline” for two different things. A device that does not answer discovery or reads is a communication problem, and the rest of this guide covers that. A device that answers but has a point showing Fault or an impossible value (a zone temperature of -40, say) has a sensor or wiring problem. If you can open the device and read its points, it is not offline.
Work through these in order
1. Are you on the same subnet?
Controls equipment normally sits on its own subnet or VLAN. A phone on guest Wi-Fi or the office network never hears the device, so everything looks offline. When a scan finds nothing, Easy BACnet shows the phone's IP address; compare it with the controller's. If they differ, see what is a subnet, and why doesn't the switch give me the right one? A device on another subnet needs a BBMD or a direct add by IP (step 3).
2. Scan again
Scan traffic gets dropped, especially over Wi-Fi. A device that appears in one scan and not the next is usually this. The app keeps asking through the scan, but a second scan or a wired connection settles it.
3. Add it directly by IP
Menu → Add device by IP… goes straight to the device's address. If it answers, it is online and the scan just isn't reaching it: it's on another subnet, or the Wi-Fi is blocking scan traffic. If it does not answer, ping it from the No devices found screen: no reply means a network problem, a reply means the device's BACnet settings. Then carry on.
4. Is UDP 47808 blocked, or is the device on another port?
An access list between VLANs, or a device set to 47809 or another non-standard port, keeps it unreachable even from the right subnet. You can type the port in when adding by IP; the access list is a question for the network team. See what port does BACnet use?
5. Does another device share its Device ID or MAC?
Two devices with the same Device Instance, or two MS/TP controllers with the same MAC on one trunk, answer for each other. The symptom is a device that drops in and out, or whose name and IP change between scans; a duplicate MAC also slows or breaks the whole trunk. The scan summary warns about both. See duplicate Device IDs.
6. It really is down
Lost power, a pulled patch cable, a dead switch port, or an MS/TP trunk that has gone quiet behind its router (open wire, missing termination, a shorted device; see the MS/TP checklist). If routed devices come and go across several trunks, ask whether two routers share a network number. Check the controller's power and link lights. A ping from a laptop on the same network shows whether the IP side is alive: see is my BACnet device online?
Check it on your own network. Easy BACnet is a free Android app that finds BACnet IP devices and reads their points in a couple of taps.
Get it on Google Play