Anycast addresses are like a cross between unicast and multicast addresses.
These addresses are designed for commonly used services such as DNS. Unicast sends
packets to one speci?¬?c device with one speci?¬?c address, and multicast sends a packet to
every member of a group. Anycast addresses send a packet to any one member of the group
of devices with the anycast address assigned.
For ef?¬?ciency, a packet that is sent to an anycast address is delivered to the closest
interface??”as de?¬?ned by the routing protocols in use??”that is identi?¬?ed by the anycast
address, so anycast can also be thought of as a ???one-to-nearest??? type of address. Anycast
addresses are syntactically indistinguishable from global unicast addresses because anycast
addresses are allocated from the global unicast address space.
Several basic types of IPv6 unicast addresses exist: global, reserved, private (link-local and
site-local), loopback, and unspeci?¬?ed. The sections that follow describe these address
types in greater detail.
NOTE There is little experience with widespread, arbitrary use of Internet anycast
addresses, and there are some known complications and hazards when using them in their
full generality.
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