For what two purposes does the Ethernet protocol use physical addresses?（）

A、to uniquely identify devices at Layer 2

B、to allow communication with devices on a different network

C、to differentiate a Layer 2 frame from a Layer 3 packet

D、to establish a priority system to determine which device gets to transmit first

E、to allow communication between different devices on the same network

F、to allow detection of a remote device when its physical address is unknown