The IPv6 addressing architecture includes a unicast interface
identifier that is used in the creation of many IPv6 addresses.
Interface identifiers are formed by a variety of methods. This
document clarifies that the bits in an interface identifier have no
meaning and that the entire identifier should be treated as an opaque
value. In particular, RFC 4291 defines a method by which the
Universal and Group bits of an IEEE link-layer address are mapped
into an IPv6 unicast interface identifier. This document clarifies
that those two bits are significant only in the process of deriving
interface identifiers from an IEEE link-layer address, and it updates
RFC 4291 accordingly.