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What does anycast mean, when used in sentences like this?

Akamai is an anycast provider.

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What is it ?

In Internet Protocol Version 6 (IPv6), anycast is communication between a single sender and the nearest of several receivers in a group. The term exists in contradistinction to multicast, communication between a single sender and multiple receivers, and unicast, communication between a single sender and a single receiver in a network.

Anycasting is designed to let one host initiate the efficient updating of router tables for a group of hosts. IPv6 can determine which gateway host is closest and sends the packets to that host as though it were a unicast communication. In turn, that host can anycast to another host in the group until all routing tables are updated.

Explanation :

On the Internet, anycast is usually implemented by using BGP to simultaneously announce the same destination IP address range from many different places on the Internet. This results in packets addressed to destination addresses in this range being routed to the "nearest" point on the net announcing the given destination IP address.

For a DNS root servers, anycast provides a service where by clients send requests to the service address and the network delivers that request to at least one, preferably the closest, instance in the root server's anycastgroup.

The Anycast scheme has two major benefits:

  • servers automatically spread the impact of an attack amongst themselves
  • no local disaster can disrupt the operation of the root server as a whole

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From Wikipedia:

Anycast is a network addressing and routing scheme whereby data is routed to the "nearest" or "best" destination as viewed by the routing topology. [...] In anycast, there is also a one-to-many association between network addresses and network endpoints: each destination address identifies a set of receiver endpoints, but only one of them is chosen at any given time to receive information from any given sender.

I really like the diagrams they use, so I'll copy them over to here. These will help if you are a visual learner like me.

Anycast

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Broadcast

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Multicast

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Unicast

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Anycast is a network addressing and routing scheme whereby data is routed to the "nearest" or "best" destination as viewed by the routing topology.

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