What a Device Instance Is
A Device Instance is a unique numeric identifier assigned to each BACnet device on a network. It is the primary way supervisory systems discover and address devices using Who-Is / I-Am services.
Valid Range
| Range | Usage |
|---|---|
| 0–4,194,302 | Assignable device instances |
| 4,194,303 | Wildcard (used in Who-Is to mean “all devices”) |
Uniqueness Requirement
[!WARNING] Device Instances must be globally unique across the entire BACnet internetwork — not just per subnet. Duplicate Device Instances cause discovery failures, data misrouting, and intermittent communication loss.
In multi-QuickServer or multi-FieldServer deployments, coordinate Device Instance assignments across all gateways before commissioning.
How It’s Used
- A supervisory system sends a Who-Is broadcast (optionally targeting a range)
- Each device responds with I-Am, which includes its Device Instance
- The supervisor builds its device list from the I-Am responses
- All subsequent communication uses the Device Instance to address the target
Common Issues
- Duplicate instances — two devices respond to the same Who-Is, causing confusion at the supervisor
- Instance not in Who-Is range — if the supervisor sends a bounded Who-Is (e.g., 100–200), devices outside that range won’t respond
- Changed after commissioning — supervisor loses track of the device if the instance changes