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IPv6 Multicast and Anycast Addressing
(Page 2 of 5)
Multicast Scopes
The notion of explicitly scoping
multicast addresses is important. Globally-scoped multicast addresses
must be unique across the entire Internet, but locally-scoped addresses
are unique only within the organization. This provides tremendous flexibility,
as every type of multicast address actually comes in several “versions”:
one that multicasts only within a node, one on the local link (local
network), one on the local site and so on. The scope also allows routers
to immediately determine how broadly they should propagate multicast
datagrams, to improve efficiency and eliminate problems with traffic
being sent outside the area for which it is intended. Figure 102
illustrates the notion of scope graphically (and it’s quite pretty,
wouldn’t you say? J)
Figure 102: IPv6 Multicast Scope This diagram shows how the notion of scope allows IPv6 multicasts to be limited to specific spheres of influence. The “tightest” scope is node-local scope, with a Scope ID value of 1. As the Scope ID value increases, the scope expands to cover the local network, site, organization, and finally, the entire Internet.
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 Key Concept: Multicast addresses are used to send data to a number of devices on an internetwork simultaneously. In IPv6 each multicast address can be specified for a variety of different scopes, allowing a transmission to be targeted to either a wide or narrow audience of recipient devices.
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