The last type of IPv6 address examined in this chapter is the anycast address (see Figure 4-13). An IPv6 anycast address is an address that can be assigned to more than one interface (typically different devices). In other words, multiple devices can have the same anycast address. A packet sent to an anycast address is routed to the “nearest” interface having that address, according to the router’s routing table.
Figure 4-13 Anycast Addresses
Anycast addresses are available for both IPv4 and IPv6, initially defined in RFC 1546, Host Anycasting Service. Anycast was meant to be used for services such as DNS and HTTP but was never really implemented as designed.
There is no special prefix for an IPv6 anycast address. An IPv6 anycast address uses the same address range as global unicast addresses. Each participating device is configured to have the same anycast address. For example, servers A, B, and C in Figure 4-14 could be DHCPv6 servers with a direct Layer 3 connection into the network. These servers could advertise the same /128 address using OSPFv3. The router nearest the client request would then forward packets to the nearest server identified in the routing table.
Figure 4-14 Example of Anycast Addressing
There are some reserved anycast address formats such as the subnet-router anycast address defined in RFC 4291 and RFC 2526. IPv6 anycast addressing is still somewhat in the experimental stages and beyond the scope of this book.