Discover BACnet/IP devices on a subnet with Who-Is/I-Am and build a device inventory of instance numbers, addresses and descriptions

Documented steps

1. Install bacpypes3 (pip install bacpypes3) and build an application: args = SimpleArgumentParser().parse_args(); app = Application.from_args(args). SimpleArgumentParser exposes --name, --instance, --network, --address, --vendoridentifier, --foreign, --ttl and --bbmd for local device identity and BBMD/foreign-device registration.
2. Send the broadcast: i_ams = await app.who_is(low_limit, high_limit) to scope the request to an instance-number range. From the interactive shell instead: python3 -m bacpypes3, then 'whois [address [low_limit high_limit]]'.
3. For each I-Am response, i_am.pduSource gives the device's BACnet Address and i_am.iAmDeviceIdentifier gives its ObjectIdentifier ('device', instance). Record both as the inventory row key.
4. Enrich each row with await app.read_property(device_address, device_identifier, 'description') and 'object-name', wrapping each call in try/except ErrorRejectAbortNack so a constrained device does not abort the whole scan.
5. Access control: BACnet/IP has no native authentication. There is no API key or credential to supply. Restrict who can reach the network via VLAN/subnet segmentation, firewalling UDP 47808 (0xBAC0), and controlling BBMD Broadcast Distribution Table and Foreign Device Table entries.
6. For devices on other IP subnets, register as a foreign device with --foreign <bbmd-addr> --ttl <seconds>, or discover routers first with the 'wirtn' (Who-Is-Router-To-Network) and 'irt' shell commands.
7. Persist the inventory keyed on (device instance, address); device instance numbers are required to be unique across the entire BACnet internetwork, so a duplicate indicates a misconfigured device rather than a second legitimate row.
8. Always close the application in a try/finally with app.close().
9. Official documentation verified 2026-07-30: https://raw.githubusercontent.com/JoelBender/BACpypes3/main/samples/discover-devices.py | https://bacpypes3.readthedocs.io/en/latest/samples/index.html | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/argparse.py