What Is a CAN Identifier?

Back to Blog

11-Bit Standard and 29-Bit Extended Frames

Classic CAN defines two frame formats that differ only in identifier length. The base frame, from the original CAN 2.0A specification, carries an 11-bit identifier, giving 2048 possible values before reserved ranges. The extended frame, from CAN 2.0B, carries a 29-bit identifier, giving over half a billion values. A single bit in the frame, the identifier extension bit, tells receivers which format is on the wire, and both formats can coexist on the same bus.

The reason two formats exist is history plus scale. Eleven bits were plenty for a car's original body electronics, but higher-layer protocols that need to pack routing information into the identifier itself outgrew that space. J1939, for instance, uses the full 29-bit extended identifier to encode a priority, a parameter group number, and a source address all inside the arbitration field, which is impossible in 11 bits. CANopen, by contrast, lives comfortably inside the 11-bit standard identifier because it addresses a modest number of nodes.

The Identifier Names the Message, Not the Node

The most important conceptual point about a CAN identifier is that it is not a destination address. When a node transmits, it does not aim the frame at another node; it broadcasts a message labelled with an identifier that means something agreed by the whole network, such as engine speed or coolant temperature. Every node on the bus receives every frame and its controller applies acceptance filters to decide which identifiers to pass up to software and which to ignore in hardware. This is content addressing, and it is why one sensor's reading can feed many consumers with a single frame.

Because the identifier also governs arbitration on the CAN bus, its numeric value is never arbitrary. A lower identifier is a higher-priority message, so identifier assignment is simultaneously a naming decision and a priority decision. Two producers must never share an identifier, or their frames would collide during arbitration and corrupt each other, which is why higher-layer protocols hand out identifier ranges systematically rather than letting integrators pick numbers freely.

This model is the opposite of a polled fieldbus like Modbus, where a unit ID names the target device and a master asks it for data. On CAN there is no master doing the asking; producers publish and consumers filter. Recognizing that difference is what stops people from hunting for a nonexistent destination field when they first read a CAN trace.

Reading an Identifier Off the Bus

On a bus analyzer or logging tool, identifiers are usually shown in hexadecimal, and the display will tell you whether each frame is standard or extended. An 11-bit identifier fits in three hex digits (0x000 to 0x7FF), while a 29-bit identifier needs up to eight (0x00000000 to 0x1FFFFFFF). If you see eight-digit identifiers, you are almost certainly looking at an extended-frame protocol such as J1939 or NMEA 2000, and the raw number can be decoded further into its embedded fields.

A frequent point of confusion is that the same physical parameter can appear under different identifiers on different vehicles or machines, because the mapping from identifier to meaning is defined by the higher-layer protocol or the manufacturer, not by CAN itself. Classic CAN is only the transport; the meaning of identifier 0x0CF00400 comes from the J1939 database, and the meaning of a CANopen frame comes from the node's device profile and its object dictionary. Without that layer, an identifier is just a priority tag on an anonymous block of bytes.

When you bring CAN data into a gateway for monitoring, the identifier is the key you map against. You tell the gateway which identifiers to capture and how to interpret their payload, then it publishes named tags upward. Getting the identifier and frame format right is the first thing to verify, because a gateway configured for 11-bit frames will silently ignore a device that only speaks in 29-bit extended frames.

Frequently Asked Questions