anycast

Anycast is a network addressing and routing technique in which multiple hosts or service endpoints share the same IP address or service identifier. Packets addressed to that address are routed to the nearest or topologically closest instance, as determined by the routing system, and the first responding endpoint provides the service. Anycast differs from unicast, where a unique address maps to a single endpoint, and from multicast, where a single address can deliver to multiple receivers regardless of proximity.

In practice, anycast is typically implemented with global routing protocols such as BGP, where multiple sites advertise the same address prefix. Routers forward traffic toward the path with the lowest cost to a reachable instance. Endpoints run identical services and often employ health checks or monitoring to prevent unreachable sites from attracting traffic. Load distribution arises from the routing topology and policies rather than explicit per-request load balancing.

Common uses include distributed DNS services (for example root and resolver servers), content delivery networks, and DDoS mitigation strategies. By serving requests from a nearby location, anycast can reduce latency and improve availability. It also helps absorb large traffic volumes by spreading load across multiple sites and avoiding a single point of congestion or failure.

Limitations and considerations include deployment complexity, sensitivity to global routing changes, potential for uneven load distribution, and troubleshooting difficulties. Because the selected endpoint depends on routing decisions rather than client location, a client may not always reach the geographically closest site, and failures can cause traffic to reroute quickly, potentially causing instability without proper health checks and consistent service state.