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
for some reason, the protocol automatically selects the second closest server, then
the third one, and so forth.
An IPv6 address is made up of eight four-digit elds, each representing 16 bits, written
in hexadecimal notation. They are separated by colons (:). Any leading zero bytes
within a given eld may be dropped, but zeros within the eld or at its end may not.
Another convention is that more than four consecutive zero bytes may be collapsed
into a double colon. However, only one such :: is allowed per address. This kind of
shorthand notation is shown in Example 13.3, “Sample IPv6 Address” (page 233), where
all three lines represent the same address.
Example 13.3:
Sample IPv6 Address
fe80 : 0000 : 0000 : 0000 : 0000 : 10 : 1000 : 1a4
fe80 : 0 : 0 : 0 : 0 : 10 : 1000 : 1a4
fe80 : : 10 : 1000 : 1a4
Each part of an IPv6 address has a dened function. The rst bytes form the prex and
specify the type of address. The center part is the network portion of the address, but
it may be unused. The end of the address forms the host part. With IPv6, the netmask
Basic Networking 233
