RFC 4193 defines the unique local address. Unique local addresses (ULAs) are designed for use in local networks and are not routable on the Internet. They substitute the deprecated site-local addresses. Unique local IPv6 addresses have a globally unique prefix. This global unique prefix is well known to allow for easy filtering at site boundaries.
As shown in Figure 2-4, the format of the unique local address is an FP of 1111 110 (FC00::/7) followed by the Global ID field, followed by the Subnet ID field and then the 64-bit Interface Identifier (ID) field. The bit labeled L is set to 1 if the prefix is locally assigned and a setting of 0 has not been defined.
Figure 2-4 IPv6 Unique Local Address
Global Aggregatable IPv6 Address
Global aggregatable unicast addresses are a type of global unicast address that allows the aggregation of routing prefixes. This aggregation makes it possible to reduce the number of routes in the global routing table. These addresses are used in links to aggregate (summarize) routes upward to the core in large organizations or to ISPs. Global aggregatable addresses are identified by a fixed prefix of 2000:/3. As shown in Figure 2-5, the format of the global aggregatable IPv6 address is a Global Routing Prefix field starting with binary 001, followed by the Subnet ID field and then the 64-bit Interface ID field. The device MAC address is normally used as the interface ID.
Figure 2-5 IPv6 Link-Local Address Format
IPv4-Compatible IPv6 Addresses
IPv4-compatible IPv6 addresses were defined to assist in the transition to IPv6 but have been deprecated. Such an address begins with 96 binary 0s (six 16-bit groups) followed by the 32-bit IPv4 address, as in 0:0:0:0:0:0:130.100.50.1, or just ::130.100.50.1. IPv4-compatible IPv6 addresses have been deprecated because updated transition mechanisms no longer require this format.
IPv4-Mapped IPv6 Addresses
RFC 4038 defines a second type of IPv6 address that holds an embedded IPv4 address. This format contains 80 bits of 0s, 16 bits of 1s, and then the 32-bit IPv4 address. This type of address is used for IPv4 clients to reach IPv6 applications. Here is an example:
::FFFF:0:0/96
IPv6 Anycast Addresses
An IPv6 anycast (one-to-nearest) address identifies a set of devices. There is no allocated prefix to identify anycast addresses. An anycast address is allocated from a set of global unicast addresses. These destination devices should share common characteristics and are explicitly configured for anycast.
You can use the anycast address to identify a set of routers or servers in an area. When a packet is sent to the anycast address, it is delivered to the nearest device, as determined by the routing protocol. Network nodes to which the anycast address is assigned must be explicitly configured to recognize that the address is an anycast address.
An example of the use of anycast addresses is to assign an anycast address to a set of servers, such as one in North America and another in Europe. Users in North America would be routed to the North American server, and those in Europe would be routed to the European server.
You cannot use an anycast address as a source address. Also, you must explicitly configure nodes to which the anycast address is assigned to recognize the anycast address.
