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The BACnet Device ID or instance number is one of the most commonly misunderstood terms in BACnet to newcomers.This is perhaps because many developers come to BACnet from other protocols such as Johnson's N2 which does not have the extra complexity of an address resolution protocol. The term "Device ID" is short for Device Identifier, and is shorthand for the Instance Number a portion of the BACnet Device object Object_Identifier property. The Device ID is used for identifying the device in the network, commonly referred to as the device "binding". I like to tell people the device ID is like the devices social security number. No two devices can have the same instance number.The Device ID is a value that is configured by someone or the HAVC system at the site when the BACnet device is installed.This number can be auto generated in most HVAC systems which can a blessing and a curse.

The BACnet standard says that "each object within a single BACnet Device shall have a unique value for the Object_Identifier property. The standard specifies that the instance number range is 22-bits: 0-4194303. It also states that "No object shall have an Object_Identifier with an instance number of 4194303."
 
When I'm planning a BACnet network I use the network number (161) + router number (01)+ the devices mac address (04) for the instance number(1610104). If you stick to a routine instance scheme and document your devices even on the largest site with multiple BACnet manufactures adding devices will be a snap. I use an excel spread sheet I call a room schedule to keep track the instance number in my BACnet network.
 
Hvac room schedule