RFC 4291:IP Version 6 Addressing Architecture
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64-bit


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... addresses other than those that start with binary 000 have a 64-bit interface ID field (i.e., n + m = 64), formatted as described in Section 2.5.1. Global Unicast ...
... addresses starting with a binary value other than 000 (and therefore having a 64-bit interface ID field) can be found in [GLOBAL ...


... IEEE, "Guidelines for 64-bit Global Identifier (EUI-64) Registration Authority ...



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