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IPv6 Addressing Overview: Addressing Model and Address Types
(Page 2 of 2)
IPv6 Address Types
One important change in the addressing
model of IPv6 is the address types supported. IPv4 supported
three address types: unicast, multicast and broadcast. Of these, the
vast majority of actual traffic was unicast. IP multicast
support was not widely deployed until many years after the Internet
was established, and continues to be hampered by various issues. Use
of broadcast in IP had to be severely restricted for performance reasons
(we don't want any device to be able to broadcast across the entire
IPv6 also supports three address
types, but with some changes:
- Unicast Addresses: These are standard
unicast addresses as in IPv4, one per host interface.
- Multicast Addresses: These are addresses
that represent various groups of IP devices: a message sent to a multicast
address goes to all devices in the group. IPv6 includes much better
multicast features and many more multicast addresses than IPv4. Since
multicast under IPv4 was hampered in large part due to lack of support
of the feature by many hardware devices, support for multicasting is
a required, not optional, part of IPv6.
- Anycast Addresses: Anycast addressing
is used when a message must be sent to any member of a group, but does
not need to be sent to them all. Usually the member of the group that
is easiest to reach will be sent the message. One common example of
how anycast addressing could be used is in load sharing amongst a group
of routers in an organization.
Key Concept: IPv6 has unicast and multicast addresses like IPv4. There is, however, no distinct concept of a broadcast address in IPv6. A new type of address, the anycast address, has been added to allow a message to be sent to any one member of a group of devices.
Implications of the Changes to Address Types in IPv6
Broadcast addressing as a distinct
addressing method is gone in IPv6. Broadcast functionality is implemented
using multicast addressing to groups of devices. A multicast group to
which all nodes belong can be used for broadcasting in a network, for
An important implication of the creation
of anycast addressing is removal of the strict uniqueness requirement
for IP addresses. Anycast is accomplished by assigning the same IP address
to more than one device. The devices must also be specifically told
that they are sharing an anycast address, but the addresses themselves
are structurally the same as unicast addresses.
The bulk of the remainder of this
section focuses on unicast addressing, since it is by far the most important
and anycast addressing are given special attention separately.
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Version 3.0 - Version Date: September 20, 2005
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