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BLADE OS 5.1 Application Guide
172
Chapter 12: IPv6 Host Management BMD00136, November 2009
Unlike IPv4, a subnet mask is not used for IPv6 addresses. IPv6 uses the subnet prefix as the
network identifier. The prefix is the part of the address that indicates the bits that have fixed values
or are the bits of the subnet prefix. An IPv6 prefix is written in address/prefix-length notation. For
example, in the following address, 64 is the network prefix:
21DA:D300:0000:2F3C::/64
IPv6 addresses can be either user-configured or automatically configured. Automatically
configured addresses always have a 64-bit subnet prefix and a 64-bit interface identifier. In most
implementations, the interface identifier is derived from the switch's MAC address, using a method
called EUI-64.
IPv6 Address Types
IPv6 supports three types of addresses: unicast (one-to-one), multicast (one-to-many), and anycast
(one-to-nearest). Multicast addresses replace the use of broadcast addresses.
Unicast Address
Unicast is a communication between a single host and a single receiver. Packets sent to a unicast
address are delivered to the interface identified by that address. IPv6 defines the following types of
unicast address:
Global Unicast address: An address that can be reached and identified globally. Global Unicast
addresses use the high-order bit range up to FF00, therefore all non-multicast and
non-link-local addresses are considered to be global unicast. A manually configured IPv6
address must be fully specified. Autoconfigured IPv6 addresses are comprised of a prefix
combined with the 64-bit EUI. RFC 4291 defines the IPv6 addressing architecture.
The interface ID must be unique within the same subnet.
Link-local unicast address: An address used to communicate with a neighbor on the same link.
Link-local addresses use the format FE80::EUI
Link-local addresses are designed to be used for addressing on a single link for purposes such
as automatic address configuration, neighbor discovery, or when no routers are present.
Routers must not forward any packets with link-local source or destination addresses to other
links.