MAC addresses are globally unique addresses defined by the IEEE to uniquely identify the physical address of a network hardware device. The IEEE has two MAC address standards, the 48-bit EUI-48™ and the 64-bit EUI-64™. Both of these node addresses consists of two parts: a unique OUI (Organizational Unique Identifier) issued by the IEEE and a unique EI (Extension Identifier). The OUI is generally purchased by different organizations directly from the IEEE and are globally unique for each organization. These organizations are then responsible for ensuring that a unique EI will go into every device they use or ship, and, in doing so, ensure a globally unique EUI node address.