As shown in MDIO Bus System, two PHY devices are attached to the MDIO bus. Each of these has a different physical address. To address the intended PHY, its physical address should be known by the MDIO master (in this case, an Ethernet MAC) and placed into the PHYAD field of the MDIO frame (see MDIO Transactions).
The PHYAD field for an MDIO frame is a 5-bit binary value capable of addressing 32 unique addresses. However, every MDIO slave must respond to physical address 0. This requirement dictates that the physical address for any particular PHY must not be set to 0 to avoid MDIO contention. Physical Addresses 1 through to 31 can be used to connect up to 31 PHY devices onto a single MDIO bus.
Physical Address 0 can be used to write a single command that is obeyed by all attached PHYs, such as a reset or power-down command.