What Network-Number-Is Is
Network-Number-Is is a network-layer message in BACnet, defined in Clause 6.4.20 of ANSI/ASHRAE 135-2024. A device uses it to announce the local network number to other devices on the same segment. It is carried in an NPDU (Network Protocol Data Unit, the network-layer packet) rather than as an application service, so it is unacknowledged. It is the answer to a What-Is-Network-Number query, and a router also emits it on its own so devices can learn or verify the number of the network they are attached to.
Like the other network-layer messages, it operates below the object and property layer. It concerns the identity of the network itself, not object data.
How It Works
The message carries a 2-octet network number and a one-octet flag that marks the number as either configured (set deliberately on the sender) or learned (received from an earlier Network-Number-Is). It is always sent with a local broadcast address and is never routed; devices ignore any copy that carries routing address fields or arrives by unicast. A device that does not know its own network number adopts a configured number it hears, and adopts a learned number only if it has never heard a configured one. Routers that know their network numbers broadcast a configured Network-Number-Is out each port at startup so the devices on each segment learn the correct number.
Where You’ll See It
- Segment identification. A device that comes up not knowing its network number learns it from a router’s Network-Number-Is broadcast.
- Answering a query. After a device broadcasts What-Is-Network-Number, a device that knows the number replies with Network-Number-Is.
- Conflict detection. A device configured with a network number that hears a conflicting configured number can report the mismatch, which is how a mis-numbered segment is caught.
Network-Number-Is Error Codes
Network-Number-Is is an unacknowledged network-layer message, so it has no application-layer error response. Trouble appears as a wrong or missing network number, not as an error code. What to keep in mind:
- A configured announcement overrides a learned one; if a device shows the wrong number, look for a router announcing a configured value that differs from what you expect.
- Two devices announcing different configured numbers for the same segment is a configuration conflict to resolve at the routers.
- The message is never routed, so a device on the wrong side of a router will not learn a segment’s number from it; it learns only from its own segment.
Profiles That Require It
The network-layer messages are the router’s job. Sending Network-Number-Is, including the startup announcement out each port, is part of the router configuration capability NM-RC-B, which is required of the router profile B-RTR. Non-routing devices may answer a query but rely on routers for the authoritative number.
What Chipkin Provides
The CAS BACnet Stack supports Network-Number-Is, both announcing a configured network number and answering a What-Is-Network-Number query. Talk to us about adding BACnet client or server support to your device.