As shown in This Figure, 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 physical address (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. Every MDIO slave must respond to physical address 0. This is a protocol bridging core. In many applications, it is expected that the external slave connected to the RGMII interface would respond to the physical address 0. This core should not respond to block that transaction. This core should respond only to the physical address assigned to it. This requirement dictates that the physical address for any particular physical address (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.