UUID

What Is a UUID in BLE?

A Universally Unique Identifier (ATT">UUID) is the addressing scheme that identifies every service, characteristic, and descriptor in the GATT hierarchy. When a client discovers a BLE device's attribute database, it reads UUIDs to understand what data the device offers and how to interact with it.

16-Bit vs. 128-Bit UUIDs

The Bluetooth SIG defines a base UUID: 00000000-0000-1000-8000-00805F9B34FB. All SIG-adopted services and characteristics are allocated a 16-bit shorthand that maps into this base. For example, the Heart Rate Service UUID 0x180D expands to 0000180D-0000-1000-8000-00805F9B34FB. This shorthand saves bandwidth during service discovery because the 16-bit form requires only 2 bytes on the wire versus 16 bytes for a full 128-bit UUID.

Custom (vendor-specific) services must use a 128-bit UUID that does not collide with the SIG base range. The standard approach is to generate a random UUID v4 and use it as a project-wide base, then vary one or two octets for different services and characteristics within the same product.

SIG-Assigned Numbers

The Bluetooth SIG maintains the Assigned Numbers document, which catalogs all standardized UUIDs. Key ranges include:

Using SIG-defined UUIDs wherever applicable improves interoperability because generic BLE tools and mobile operating systems can automatically label and parse these attributes.

UUID in Advertising

When a device advertises, it can include a list of service UUIDs in the advertising payload to help scanners filter devices by capability before connecting. The advertising data types for this purpose include Complete List of 16-bit Service UUIDs (type 0x03), Incomplete List (type 0x02), and corresponding 128-bit variants. Including UUIDs in advertising consumes valuable payload space (31 bytes in legacy advertising, more with extended advertising), so devices typically advertise only their primary service UUID.

Collision Avoidance

Although a randomly generated 128-bit UUID has an astronomically low collision probability (1 in 2^122), developers should still document and register their UUIDs internally. Some organizations maintain a UUID registry per product line. Reusing another vendor's custom UUID can cause subtle interoperability failures where a client connects to the wrong device type and misinterprets characteristic data.