Ethernet Address Internal Structure

The second-to-lowest-order bit of a physical Ethernet address indicates whether that address is believed to be globally unique or if it is only locally unique; this is known as the Universal/Local bit. For real Ethernet physical addresses, the multicast and universal/local bits of the first byte should both be 0.

When (global) Ethernet IDs are assigned to physical Ethernet cards by the manufacturer, the first three bytes serve to indicate the manufacturer. They are allocated by the IEEE, and are officially known as organizationally unique identifiers. These can be looked up at any of several sites on the Internet to identify the manufacturer associated with any given Ethernet address; the official IEEE site is standards.ieee.org/develop/regauth/oui/public.html (OUIs must be entered here without colons). As long as the manufacturer involved is diligent in assigning the second three bytes, every manufacturerprovided Ethernet address should be globally unique. Lapses, however, are not unheard of.

Ethernet addresses for virtual machines must be distinct from the Ethernet address of the host system, and may be (eg with so-called “bridged” configurations) as visible on the LAN as that host system’s address. The first three bytes of virtual Ethernet addresses are often taken from the OUI assigned to the manufacturer whose card is being emulated; the last three bytes are then either set randomly or via configuration. In principle, the universal/local bit should be 1, as the address is only locally unique, but this is often ignored. It is entirely possible for virtual Ethernet addresses to be assigned so as to have some local meaning, though this appears not to be common.