What is a BACnet Instance Number?

A BACnet Instance Number Is a Globally-unique Address

An oft misunderstood and neglected concept is the BACnet instance number of a BACnet device. Neglecting this number at installation might not bite you today, but it remains as technical debt that complicates future work. It’s simply an address number that identifies that device uniquely on the entire interconnected BACnet network. No other device anywhere on the network may have the same BACnet instance number, even if it’s on a different bus in a different building. If they can see each other, they can’t have the same address. This is in contrast to the MS/TP address, which doesn’t have to be globally unique.

More Technical

The instance number is an unsigned decimal number that can range from 0 to 4,194,302. Every device on a BACnet network gets an instance number, and two devices must not have the same number. Devices on different physical network segments that can talk to each other are on the same BACnet network – they can’t have the same instance numbers!

Don’t neglect the instance number uniqueness just because you’re installing MS/TP devices. MS/TP devices also have MAC addresses and need to communicate to BACnet/IP devices through a gateway, but they still have unique BACnet instance numbers that cannot be duplicated anywhere else on the network.

If you have duplicate instance numbers on your network you’ll have a similar problem that you’d see if you have duplicate IP addresses. Some devices on the network will talk to one and not the other, and the devices themselves will have unreliable communication with the overall network.

To avoid problems, start your installation with a clear numbering convention. Here’s an example one I like to use:

1 – 23 – 03 – 23 = 1230323

SITE/Building – Floor – Bus/Network Number – MAC Address

I keep to this even if it’s only for one building as it makes future expansion easy. The numbers are a bit limited, you can only expand to 4 sites/buildings this way. If you have a broader campus/etc. you’d probably want to arrange a different numbering convention. Just spend the time to think carefully about it and you’ll save yourself in the long run!