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For all IEEE 802 interface types (for example, Ethernet and FDDI interfaces), Interface IDs in
the modified EUI-64 format are constructed in the following way:
The first three octets (24 bits) are taken from the Organizationally Unique Identifier (OUI) of the
48-bit link-layer address (the MAC address) of the interface, the fourth and fifth octets (16 bits)
are a fixed hexadecimal value of FFFE, and the last three octets (24 bits) are taken from the
last three octets of the MAC address. The construction of the interface ID is completed by
setting the universal/local (U/L) bit--the seventh bit of the first octet--to a value of 0 or 1. A
value of 0 indicates a locally administered identifier; a value of 1 indicates a globally unique
IPv6 interface identifier.
Take MAC address 0012:0B0A:2D51 as an example. Insert FFFE to the middle of the address
to get 0012:0BFF:FE0A:2D51. Then set the U/L bit to 1 to obtain an interface ID in EUI-64
format as 0212:0BFF:FE0A:2D51.
IPv6 unicast address can be classified into several types, as shown in Table 4-1. The two
most common types are introduced below:
Global unicast address
A Global unicast address is an IPv6 unicast address that is globally unique and is routable on
the global Internet.
Global unicast addresses are defined by a global routing prefix, a subnet ID, and an interface
ID. The IPv6 global unicast address starts with binary value 001 (2000::/3). The global routing
prefix is a value assigned to a site (a cluster of subnets/links) by IANA. The subnet ID is an
identifier of a subnet within the site.
The figure below shows the structure of a global unicast address.
Figure 4-8 Global Unicast Address Format
Link-local address
A link-local address is an IPv6 unicast address that can be automatically configured on any
interface using the link-local prefix FE80::/10 (1111 1110 10) and the interface identifier in the
modified EUI-64 format. Link-local addresses are used in the neighbor discovery protocol and
the stateless autoconfiguration process. Nodes on a local link can use link-local addresses to
communicate. The figure below shows the structure of a link-local address.
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