Every Bluetooth LE device is identified by a unique 48-bit address. Bluetooth addresses are categorized as either public addresses or random addresses. Random addresses can further be classified into static or private addresses, depending on whether they change or not. Furthermore, private addresses can either be resolvable or non-resolvable.

All Bluetooth LE devices must have either a public address or a random static address, the latter of which is used by default in Nordic devices. Optionally, they can also have resolvable or non-resolvable private addresses, usually added for privacy reasons.

Public address

A public address is a fixed address that is programmed into the device at the manufacturer. It must be registered with the IEEE registration authority, and it’s globally unique to that device and cannot change during the lifetime of the device. There is a fee associated with obtaining this type of address.

Random address

A random address is much more commonly used, as it does not require registration with the IEEE and can be manually configured by the user. This is also the default address type in the samples in the nRF Connect SDK. It is either programmed within the device or created during runtime. You can either have a static or a private address.

Random static address

A random static address can be allocated and fixed throughout the lifetime of the device. It can be altered at bootup, but not during runtime. This is a low-cost alternative to a public address because you don’t need to register it.

As mentioned above, all Bluetooth LE devices must use either a public address or a random static address, where the latter is far more common.

Random private address

A private address can be used when a device wishes to protect its privacy. This is an address that changes periodically and is used to hide the device’s identity and to deter device tracking.

A random private address can be resolvable or non-resolvable, and are described below.

Resolvable random private address

A resolvable private address is, true to its name, resolvable as intended listeners have a pre-shared key by which they can figure out the new address every time it changes. The pre-shared key is the Identity Resolving Key (IRK) and is used both to generate and to resolve the random address.

The random address is basically just used by the peer to be able to resolve the actual address of the Bluetooth LE device, which is still either the public or the random static address. The IRK allows the peer to translate the random private address into the device’s real Bluetooth LE address.

Non-resolvable random private address

A non-resolvable private address is not resolvable by other devices and is only intended as a way to prevent tracking. This type of address is not commonly used.

To summarize there are four different types of addresses:

• Public address: Programmed into the device by the manufacturer, and is registered with the IEEE.
• Random static address: Configurable at boot up and is fixed through the lifetime of the device. Does not need to be registered with the IEEE, and is a common alternative to a public address.
• Resolvable random private address: (optional) An address that changes periodically, but is resolvable by means of a pre-shared key.
• Non-resolvable random private address: (optional) An address that changes periodically and is not resolvable.