The last 64 bits of an IPv6 address are what is known as the interface identifier. IPv6 unicast addresses are made up of a prefix followed by an Interface Identifier (IID), the last 64 bits. According to a recent RFC on the subject, these identifiers are formed through varying methods. The RFC, which can be read in its entirety here, elaborates on the idea that the bits in the interface identifier should be “treated as an opaque value”, and have no stand-alone significance.

This finding comes to a conclusion that the value of the "u" bit in IIDs contains no significant meaning. As stated in the document, “In the case of an IID created from a MAC address according to RFC 4291, its value is determined by the MAC address, but that is all.” The U and G bits are found to have little significance in relation to their originally believed “purpose”, but may present other details of importance.

This RFC makes a change to RFC 4291, by indicating that the Universal and Group bits of an IEEE link-layer address are merely significant in the act of gleaning interface identifiers from that IEEE link-layer address.