If your device is privacy-enabled, the IRK is needed to continue to follow the device when it changes its Bluetooth LE address. You should provide it in hexadecimal format (starting with 0x, big endian). You must do this before the device enters encryption. If the entered key is shorter than 16 bytes, you will get an error. When selecting a device that can be resolved with the IRK, the sniffer continues to follow any Bluetooth LE addresses that also resolves with the IRK. If the current LE address of the device is unknown, the device can be followed by selecting Follow IRK in the device list.