What is Anycast?

Anycast is a specific addressing mode in computer networks. It is applied in Internet Protocol Version 6 (IPv6) and describes the communication between a single sender and the nearest receiver in a group.

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Explanation Anaycast


A definition of what anycast is

Anycast is a routing scheme in which a common address is assigned to a group of computers. However, the sender only reaches the computer that can be reached via the shortest route.

For the implementation of anycast, several servers of the same type are distributed over spatially separated IP networks. The computers receive the same IP address. They are accessible via a routing protocol. If a server fails or becomes unavailable, this route is no longer available. Instead, the sender routes all subsequent packets to another server. Since the servers are accessible via the same IP address, communication for the client works like a unicast communication (see below).


Advantages of Anycast

High Availability

The failure of one or more servers can be compensated by anycast.

Load Distribution

Unnoticed by the sender, the load of the requests can be distributed among the different servers by always delivering the packets to the nearest server.

Shorter Access Times

The possible geographical proximity of the respective servers to the sender shortens the access time.


Alternative Routing Schemes

In addition to anycast there are other routing schemes that work differently.


Unicast is the standard in data transmission where there is exactly one sender and one receiver. Unicast routing occurs, for example, when a user accesses a website hosted on a single server.

For the client there is no difference compared to unicast when using anycast, because the anycast servers can be reached via the same IP address. In most cases they also have a unicast address so that they can be addressed individually for administration purposes.


In multicast, an endpoint sends data which is forwarded to the requesting devices via nodes. Multicast is used for IPTV transmissions, among others, to save bandwidth. As a result, not every device has to request the stream individually, but the stream is split and distributed onwards.


In broadcast, a single endpoint sends data to all endpoints in a broadcast domain. Among other things, television stations use this scheme for video and audio distribution.


Geocast works in large part like multicast: An endpoint sends data to a variety of devices. However, geocast is limited to a specific geographical region. It is therefore mainly used for regional messages and services. The geographical area is determined by means of geo-routers and GPS data.


Applications of Anycast in Practice

In practice, anycast has been shown to be particularly useful in the following scenarios:

  • Critical infrastructure elements: Decentralization using anycast protects critical infrastructure from failures. For example, this applies to the domain name system root servers (DNS root servers) which manage all addresses on the internet.

  • Protection against DDoS attacks: Distributed Denial of Service (DDoS) attacks attempt to restrict the accessibility of websites or other online services or make them completely inaccessible. This is done via an vast number of automated bot requests, which overload the infrastructure. Anycast offers a certain measure of protection here, because the requests can be forwarded to other servers on a large scale.


Anycast: Things you need to know

Anycast is a routing scheme that allows companies and administrators to make multiple servers accessible from the same IP address. The advantages of anycast include high availability, good load balancing and shorter access times.

As a result, in combination with other measures, anycast enables the flexible distribution of requests as well as the high-performance delivery of websites or applications.

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