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What is Anycast?
- Authors
- Name
- Skip2 Networks
- Title
- Content Manager
Anycast
Anycast is a network addressing and routing method where multiple hosts or servers, geographically distributed across different locations, are configured to advertise the same IP address. When a client initiates a connection to this Anycast IP address, the internet's routing protocols (like BGP - Border Gateway Protocol) direct the client's traffic to the "nearest" or "best" available server advertising that address. The determination of "nearest" is typically based on routing metrics such as the shortest network path (lowest hop count), lowest latency, or other routing policy considerations, rather than geographical proximity in a straight line. This creates a highly efficient system where users are automatically directed to the optimal service endpoint without needing to know the specific location of individual servers.
This technique is a cornerstone of modern internet infrastructure, particularly critical for services that benefit from global distribution, high availability, and performance. Content Delivery Networks (CDNs) extensively use Anycast to serve web content, streaming media, and applications from the closest Point of Presence (PoP) to the end-user, drastically reducing latency and improving user experience. Similarly, the global DNS root servers, which are vital for internet navigation, leverage Anycast to provide resilient and fast domain name resolution worldwide. Beyond performance, Anycast also offers significant security benefits, as it can absorb and distribute large volumes of malicious traffic during a Distributed Denial of Service (DDoS) attack across multiple locations, making it harder for attackers to overwhelm a single target.
Example
Imagine a global CDN company providing content delivery services. They might have data centers (PoPs) in New York, London, and Tokyo, and all of these PoPs are configured to advertise the same Anycast IP address, say 203.0.113.50, for their caching service.
When a user in Paris wants to access a website served by this CDN:
- Their computer sends a request to
203.0.113.50. - Internet routers, based on their routing tables and the shortest path, determine that the London PoP is the most efficient route for traffic originating from Paris.
- The user's request is then routed directly to the London PoP, and the content is served from there.
Simultaneously, a user in Sydney accessing the same content would have their request routed to the Tokyo PoP, as it's the "nearest" advertising 203.0.113.50 from their location. This ensures both users receive content quickly from a location geographically closer to them, despite requesting the same single IP address.