example, the multicast address with the prefix FF02::/16
is a multicast address with a link scope. All devices
implementing IPv6 are required to join the all-nodes
multicast group with the FF02:0:0:0:0:0:0:1 multicast
address. IPv6 routers must also join the all-routers
multicast group FF02:0:0:0:0:0:0:2. The scope on both
of these multicast addresses is link-local.
Loopback IPv6 addresses are similar to the loopback
addresses in IPv4: They are used for testing local traffic
on a device. In IPv6 there is only one address dedicated
for this purpose: 0:0:0:0:0:0:0:1 (or ::1 when using the
zero compression feature in IPv6).
The unspecified address is used for situations when a
device doesn’t have any IPv6 address assigned and needs
to populate the Source field when sending a datagram on
the network, such as when obtaining an IPv6 address
from a DHCPv6 server. This address is the all-zeros
address: 0:0:0:0:0:0:0:0 (or just :: when using the zero
compression feature in IPv6).
Anycast addresses are allocated from the unicast global
space. Their purpose is to accommodate the one-to-
nearest data traffic requirement. In this case, multiple
devices share the same unicast address, and data traffic
that is destined for the anycast address is routed through
the network to the closest device that has that anycast
address configured on one of its interfaces. In this case,
of course, all the anycast nodes have to provide a similar,
uniform service. Anycast is suitable for load balancing
and content delivery services. An example of a service
that can be implemented using anycast addresses is
Domain Name System (DNS). DNS is covered in Chapter
18 ; for the purposes of this example, you just need to
know that DNS is a critical component of the Internet
and is mainly used to resolve domain names to IP
addresses. Configuring several DNS servers on a network
with the same anycast address ensures a consistent DNS