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Anycast IPv6 addresses

Updated: January 21, 2005

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Anycast IPv6 addresses

An anycast address identifies multiple interfaces. With the appropriate routing topology, packets addressed to an anycast address are delivered to a single interface (the nearest interface that is identified by the address). The nearest interface is defined as being closest in terms of routing distance. A multicast address is used for one-to-many communication, with delivery to multiple interfaces. An anycast address is used for one-to-one-of-many communication, with delivery to a single interface.

In order to facilitate delivery to the nearest anycast group member, the routing infrastructure must be aware of the interfaces that are assigned anycast addresses and their distances in terms of routing metrics. At present, anycast addresses are used only as destination addresses and are assigned only to routers. Anycast addresses are assigned from the unicast address space. The scope of an anycast address is the scope of the type of unicast address from which the anycast address is assigned.

Subnet-router anycast address

The subnet-router anycast address is both predefined and required. It is created from the subnet prefix for a given interface. To construct the subnet-router anycast address, the bits in the subnet prefix are fixed at their appropriate values and the remaining bits are set to 0. All router interfaces attached to a subnet are assigned the subnet-router anycast address for that subnet. The subnet-router anycast address is used for communication with one of multiple routers attached to a remote subnet.

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