IPv6 global unicast address format

FIGURE 2 . 7 IPv6 global unicast address format
Global unicast addresses are required to have 64-bit interface identities in the extended universal
identifier (EUI-64) format. IPv6 uses a modified EUI-64 format to identify a unique interface on a
network segment. This modified EUI-64 is based on the Data Link layer (MAC) address of an interface.
It usually inserts the 16-bit value of 0xFFFE between the 24-bit vendor ID and the 24-bit
vendor-supplied unique extension identifier of the MAC address. Also the modified EUI-64 format
says that the u-bit, which is usually set to 0 by the manufacturer to signify a globally unique value
of the address, must be inverted, or set to 1, which indicates that the address may have a less official
value that must only be unique on a local level. This gives the administrator the freedom and flexibility
to design a locally significant addressing scheme for links, such as serial links and tunnel endpoints,
which do not have burned-in hardware addresses from which to create an interface ID.
Figure 2.8 shows how this modification would take place.
A MAC address of 0060.08D2.7B4B will be converted to the 64-bit identifier of
0260.08FF.FED2.7B4B. This identifier is then used to create an IPv6 address such as
205B:8B:CC16:6E:260:8FF:FED2:7B4B.
Site-Local Unicast Address
Site-local unicast addresses are similar in concept to the RFC 1918 Intranet address space for
IPv4 networks. These addresses can be used to restrict communication to a specific portion of
the network or to assign addresses for a network that is not connected to the global Internet
without requiring a globally unique address space. IPv6 routers will not forward traffic with
site-local source or destination addresses outside the boundary of the site’s network.
001
Global Routing Prefix Subnet ID
Provider Site
Interface ID
Host
3 bits 45 bits 16