addresses, a node needs to know a network interface that a destination link-local address is related to, as by definition all interfaces have the same link-local prefix. 3.1.6 Interface Identifier and the Modified EUI-64 The IPv6 address architecture defines that any IPv6 unicast address whose upper three bits are not zeros (i.e. a binary value 000) must have a 64-bit Interface IDentifier constructed using the modified EUI-64 format [4]. Sections 2.5.1 and 2.5.4 of RFC 4291 are very clear on this. Modified EUI-64 format-based Interface Identifiers may be derived from other unique tokens such as Institute of Electrical and Electronics Engineers (IEEE) 802 48-bit Media Access Control (MAC) identifier [9] or Non-Broadcast Multiple Access (NBMA) interface tokens [10]. See Figure 3.4 for an illustration of what the modified EUI-64 format looks like. In the figure 'C' forms the bits of the assigned company_id and 'M' forms the bits of the manufacturer selected extension identifier. To differentiate from the original IEEE managed EUI-64s and to make the system administration slightly easier, the modified EUI-64 has the universal/local 'U' bit inverted, that is the bit 70 of an IPv6 address. When the 'U' is set to 0 (zero), then the EUI-64 is non-global and managed locally. The 'G' is the individual/group bit. When creating interface identifiers using the modified EUI-64 format, care should be taken not to override the 'U' and 'G' bits' predefined meaning described in RFC 4291 and in [9]. Summarizing, when the 'U' is set to 0 (zero) the remaining 63 bits ('G' bit also included) can be of any value. For example, privacy addresses [11] makes use of this. Figure 3.5 shows how an IEEE 802 48-bit MAC identifier is 'expanded' into a modified EUI-64 format. Two octets, 0xff and Oxfe, are inserted into the middle of the modified EUI-64. More specifically the two octets are placed in bits 24 to 39. Note that in the figure the 'U' bit is set to 1 (one), which means that the 48-bit MAC identifier had its corresponding 'U' bit clear