The second step is to invert the universal/local (U/L) flag (bit 7) in the OUI portion of the address. In other words, any EUI-64 address having 0xFFFE immediately following its OUI portion can be recognized as having been generated from an EUI-48 (or MAC) address. Why 0xFFFE? As explained in the IEEE's Guidelines for EUI-64 Registration Authority, this is a reserved value which equipment manufacturers cannot include in "real" EUI-64 address assignments. The 16-bit hex value 0xFFFE is then inserted between these two halves to form a 64-bit address. To do this, we break the MAC address into its two 24-bit halves: the Organizationally Unique Identifier (OUI) and the NIC specific part. The first step is to convert the 48-bit MAC address to a 64-bit value. RFC 2373 dictates the conversion process, which can be described as having two steps. This is accomplished on Ethernet interfaces by referencing the already unique 48-bit MAC address, and reformatting that value to match the EUI-64 specification. By implementing the IEEE's 64-bit Extended Unique Identifier (EUI-64) format, a host can automatically assign itself a unique 64-bit IPv6 interface identifier without the need for manual configuration or DHCP. One of IPv6's key benefits over IPv4 is its capability for automatic interface addressing.
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