Operation Manual
servers to address hosts through multicasting—by addressing a number of hosts as
parts of a group (which is different from addressing all hosts through broadcasting
or each host individually through unicasting). Which hosts are addressed as a group
may depend on the concrete application. There are some predened groups to ad-
dress all name servers (the all name servers multicast group), for example, or all
routers (the all routers multicast group).
23.2.2 Address Types and Structure
As mentioned, the current IP protocol is lacking in two important aspects: there is an
increasing shortage of IP addresses and conguring the network and maintaining the
routing tables is becoming a more complex and burdensome task. IPv6 solves the rst
problem by expanding the address space to 128 bits. The second one is countered by
introducing a hierarchical address structure, combined with sophisticated techniques
to allocate network addresses, as well as multihoming (the ability to assign several ad-
dresses to one device, giving access to several networks).
When dealing with IPv6, it is useful to know about three different types of addresses:
Unicast
Addresses of this type are associated with exactly one network interface. Packets
with such an address are delivered to only one destination. Accordingly, unicast
addresses are used to transfer packets to individual hosts on the local network or
the Internet.
Multicast
Addresses of this type relate to a group of network interfaces. Packets with such
an address are delivered to all destinations that belong to the group. Multicast ad-
dresses are mainly used by certain network services to communicate with certain
groups of hosts in a well-directed manner.
Anycast
Addresses of this type are related to a group of interfaces. Packets with such an
address are delivered to the member of the group that is closest to the sender, ac-
cording to the principles of the underlying routing protocol. Anycast addresses are
used to make it easier for hosts to nd out about servers offering certain services
in the given network area. All servers of the same type have the same anycast ad-
dress. Whenever a host requests a service, it receives a reply from the server with
the closest location, as determined by the routing protocol. If this server should fail
366 Reference