<P> The IEEE now considers the label MAC - 48 to be an obsolete term, previously used to refer to a specific type of EUI - 48 identifier used to address hardware interfaces within existing 802 - based networking applications, and thus not to be used in the future . Instead, the proprietary term EUI - 48 should be used for this purpose . </P> <P> In addition, the EUI - 64 numbering system encompasses both MAC - 48 and EUI - 48 identifiers by a simple translation mechanism . To convert a MAC - 48 into an EUI - 64, copy the OUI, append the two octets FF - FF and then copy the organization - specified extension identifier . To convert an EUI - 48 into an EUI - 64, the same process is used, but the sequence inserted is FF - FE . In both cases, the process can be trivially reversed when necessary . Organizations issuing EUI - 64s are cautioned against issuing identifiers that could be confused with these forms . The IEEE has a target lifetime of 100 years for applications using MAC - 48 space, but encourages adoption of EUI - 64s instead . </P> <P> IPv6--one of the most prominent standards that uses a Modified EUI - 64--treats MAC - 48 as EUI - 48 instead (as it is chosen from the same address pool) and toggles the U / L bit (as this makes it easier to type locally assigned IPv6 addresses based on the Modified EUI - 64). This results in extending MAC addresses (such as IEEE 802 MAC address) to Modified EUI - 64 using only FF - FE (and never FF - FF) and with the U / L bit inverted . </P> <P> An Individual Address Block (IAB) was a 24 - bit OUI managed by the IEEE Registration Authority, followed by 12 IEEE - provided bits (identifying the organization), and 12 bits for the owner to assign to individual devices . An IAB is ideal for organizations requiring fewer than 4097 unique 48 - bit numbers (EUI - 48). IAB's have been replaced with 12 - bit "MA - S" address blocks . </P>

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