There are different types of unicast addresses:
Global unicast address
Site-local unicast address
Link-local unicast address
IPv4-mapped IPv6 address
IPv4-compatible IPv6 address
Unspecified address
The other type of unicast address, 0:0:0:0:0:0:0:1, or ::1, is the loopback address and per-forms the same function as 127.0.0.1 does in IPv4. It is used to identify a transmission sent by a node back to itself, usually for testing purposes, and should never leave the sending node. This cannot be assigned to a physical interface, and IPv6 routers do not forward traffic either sourced from or destined to this address.
An Overview of IPv6 Addressing 69
Global Unicast Address
The IPv6 aggregatable global unicast address is the equivalent to the Class A, B, or C IPv4 address.
Theoretically, a global unicast address is any address that is not one of the other named types, which accounts for 85 percent of the IPv6 address space. But IANA has been limited to allocating only aggregatable global unicast addresses, which begin with binary 001, a portion of the address known as the global unicast format prefix, which is 2000::/3 in IPv6 hexadecimal notation. This is still the largest block of assigned IPv6 addresses and represents 1⁄8 of the total address space.
The structure of global unicast addresses enables aggregation of the routing prefixes that will limit the number of routing table entries in the global routing table. Global unicast addresses are aggregated upward through an organization and eventually to the Internet service providers (ISPs). Figure 2.7 shows that global unicast addresses, which start with binary 001, are made up of a global routing prefix, followed by a subnet ID, and finally an interface ID.
F I G U R E 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 inter-face. 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 flexi-bility to design a locally significant addressing scheme for links, such as serial links and tunnel end-points, 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
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F I G U R E 2 . 8 Converting a MAC address to an EUI-64 address
The site-local unicast addresses use the prefix range FEC0::/10, which is padded with 38 0s and then appends the 16-bit subnet identifier, followed by the 64-bit interface ID. Figure 2.9 shows the format of the site-local unicast address.
F I G U R E 2 . 9 Site-local unicast address
Link-Local Unicast Address
A link-local unicast address is used in the neighbor discovery protocol and is used only on the local link network. This is used by the stateless auto-configuration process for devices to dis-cover the Data Link layer address of the network and to find and keep track of neighbors. A link-local unicast address uses the prefix range FE80::/10, which is padded with 54 0s, followed by the 64-bit interface ID. Figure 2.10 shows the format of the link-local unicast address.
F I G U R E 2 . 1 0 Link-local unicast address 1111111111111110
Vendor Identifier Unique Extension Identifier
24 bits 24 bits
VVVVVV1VVVVVVVVVVVVVVVVV
64 bits
VVVVVV0VVVVVVVVVVVVVVVVV UUUUUUUUUUUUUUUUUUUUUUUU
UUUUUUUUUUUUUUUUUUUUUUUU 16 bits
Invert
FEC0::/10
0 Subnet ID Interface ID
64 bits 16 bits
38 bits
128 bits
10 bits
FE80::/10
0 Interface ID
64 bits 54 bits
128 bits
10 bits
An Overview of IPv6 Addressing 71
IPv4-Compatible IPv6 Address
As a transition mechanism, the IPv4-compatible IPv6 address is used to tunnel IPv6 packets over an IPv4 infrastructure, without the need to preconfigure tunnels through the IPv4 network. This address type embeds an IPv4 address in the low-order 32 bits. It pads all 96 high-order bits with 0s. It is used between two interfaces that support both the IPv4 and IPv6 protocol stacks, but are separated by devices that support only IPv4, and the format is 0:0:0:0:0:0:A.B.C.D, or ::A.B.C.D, where A.B.C.D is the IPv4 unicast address. Nodes that are assigned IPv4-compatible IPv6 addresses perform automatic tunneling. Whenever a node with one of these addresses sources or receives an IPv6 packet whose next hop is over an IPv4 interface, it must encapsulate the IPv6 packet within an IPv4 packet before sending it out. Conversely, these nodes must be prepared to accept IPv4 packets with IPv6 packets encapsulated within. In addition to the information found in RFC 3513, RFC 2893 gives additional details concerning IPv4-compatible IPv6 addresses.
IPv4-Mapped IPv6 Address
This type of address also embeds an IPv4 address in the low-order 32-bits, but with 0s in only the first 80 high-order bits and 1s in the next 16 bits—bits 81 to 96. This address type is used by devices that support both IPv4 and IPv6 protocol stacks in order that they may commu-nicate with devices that support only IPv4. On the dual-stack device, an IPv6 application that is sending traffic to the IPv4 device’s IPv4-mapped IPv6 address will recognize the meaning of this type of address and send IPv4 packets—not IPv6 packets—to that destination. In other words, this type of addressing mechanism does not encapsulate IPv6 packets within IPv4 packets. Conversely, if such a node receives a pure IPv4 packet that must be forwarded into the IPv6 domain, the dual-stack node will create the IPv4-mapped IPv6 address, to be used as the IPv6-header source address, from the incoming packet’s original IPv4 source address. So any return traffic will be known by the dual-stack node to be destined for an IPv4-only inter-face, and will be forwarded as such. IPv4-mapped IPv6 addresses are even more of a transition mechanism, and their address format is ::FFFF:A.B.C.D, where A.B.C.D is the IPv4 unicast address. A common use for this type of address is when an IPv6-enabled DNS server responds to the request of a dual IPv6/IPv4 node with the IP address of an IPv4-only node. The DNS server returns the IPv4-mapped IPv6 address, and the dual node knows what to do from there.
Unspecified Address
An unspecified IPv6 address is a special address that is used as a placeholder by a device that does not have an IPv6 address. This might happen when the node requests an address from a DHCP server or when the duplicate address detection packet is sent. The format is 0:0:0:0:0:0:0:0 but can be represented by 0::0 or just ::/128. This IPv6 address cannot be assigned to any interface and should not be used as a destination address.