Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 428 — #454
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On top of this basic structure, IPv6 distinguishes between five different
types of unicast addresses:
:: (unspecified) This address is used by the host as its source address
when the interface is initialized for the first time — when the address
cannot yet be determined by other means.
::1 (loopback) The address of the loopback device.
IPv4 Compatible Addresses The IPv6 address is formed by the IPv4 ad-
dress and a prefix consisting of 96 zero bits. This type of compatibility
address is used for tunneling (see Section 21.2.3 on the next page) to
allow IPv4 and IPv6 hosts to communicate with others operating in a
pure IPv4 environment.
IPv4 Addresses Mapped to IPv6 This type of address specifies a pure
IPv4 address in IPv6 notation.
Local Addresses There are two address types for local use:
link-local This type of address can only be used in the local subnet-
work. Packets with a source or target address of this type should
not be routed to the Internet or other subnetworks. These ad-
dresses contain a special prefix (fe80::/10) and the interface
ID of the network card, with the middle part consisting of null
bytes. Addresses of this type are used during autoconfiguration
to communicate with other hosts belonging to the same subnet-
work.
site-local Packets with this type of address may be routed to other
subnetworks, but not to the wider Internet — they must remain
inside the organization’s own network. Such addresses are used
for intranets and are an equivalent of the private address space
as defined by IPv4. They contain a special prefix (fec0::/10),
the interface ID, and a sixteen bit field specifying the subnet-
work ID. Again, the rest is filled with null bytes.
As a completely new feature introduced with IPv6, each network interface
normally gets several IP addresses, with the advantage that several net-
works can be accessed through the same interface. One of these networks
can be configured in completely automatic fashion, using the MAC and
a known prefix, with the result that all hosts on the local network can be
reached as soon as IPv6 is enabled (using the link-local address). With the
MAC forming part of it, any IP address used in the world is unique.
428 21.2. IPv6 — The Next Generation Internet