<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> <P> Addresses can either be universally administered addresses (UAA) or locally administered addresses (LAA). A universally administered address is uniquely assigned to a device by its manufacturer . The first three octets (in transmission order) identify the organization that issued the identifier and are known as the organizationally unique identifier (OUI). The remainder of the address (three octets for MAC - 48 and EUI - 48 or five for EUI - 64) are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness . A locally administered address is assigned to a device by a network administrator, overriding the burned - in address . </P>

How many bits does a mac address have