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Day 19 141The current global unicast address that is assigned by the IANA uses the range of addresses thatstart with binary value 001 (2000::/3), which is one-eighth of the total IPv6 address space and isthe largest block of assigned addresses.Reserved, Private, and Loopback AddressesThe IETF reserves a portion of the IPv6 address space for various uses, both present and future.Reserved addresses represent 1/256 of the total IPv6 address space. Some of the other types ofIPv6 addresses come from this block.A block of IPv6 addresses is set aside for private addresses, just as is done in IPv4. These privateaddresses are local only to a particular link or site and therefore are never routed outside a particularcompany network. Private addresses have a first-octet value of FE in hexadecimal notation,with the next hexadecimal digit being a value from 8 to F.These addresses are further divided into two types, based on their scope:■■Site-local addresses: These are for an entire site or organization. However, the use of sitelocaladdresses is problematic and is being deprecated as of 2003 by RFC 3879. In hexadecimal,site-local addresses begin with FE and then C to F for the third hexadecimal digit. So,these addresses begin with FEC, FED, FEE, or FEF.Link-local addresses: These have a smaller scope than site-local addresses; they refer to onlya particular physical link (physical network). Routers do not forward datagrams using linklocaladdresses, not even within the organization; they are only for local communication on aparticular physical network segment. They are used for link communications such as automaticaddress configuration, neighbor discovery, and router discovery. Many IPv6 routing protocolsalso use link-local addresses. Link-local addresses begin with FE and then have a valuefrom 8 to B for the third hexadecimal digit. So, these addresses start with FE8, FE9, FEA, orFEB.Just as in IPv4, a provision has been made for a special loopback IPv6 address for testing. Theloopback address is 0:0:0:0:0:0:0:1, which normally is expressed using zero compression as ::1.The IPv6 Interface ID and EUI-64 FormatFigure 19-1 showed the format of an IPv6 global unicast address, with the second half of theaddress called the host or interface ID. The value of the interface ID portion of a global unicastaddress can be set to any value, as long as no other host in the same subnet attempts to use thesame value. However, the size of the interface ID was chosen to allow easy autoconfiguration of IPaddresses by plugging the MAC address of a network card into the interface ID field in an IPv6address.MAC addresses are 6 bytes (48 bits) in length. So to complete the 64-bit interface ID, IPv6 fills in2 more bytes by separating the MAC address into two 3-byte halves. It then inserts hex FFFE inbetween the halves and sets the seventh bit in the first byte to binary 1 to form the interface IDfield. Figure 19-2 shows this format, called the EUI-64 format (EUI stands for Extended UniqueIdentifier).