What Is Anycast? How One IP Serves the Whole World

Anycast is the clever routing trick that lets a single IP address live in dozens of data centres at once, quietly sending every visitor to the nearest copy to make the internet faster and far more resilient.

Here is a puzzle. When you look up your own address, an IP points to exactly one place: your connection. So how can a giant service answer the same IP address from London, Tokyo, and São Paulo simultaneously, sending each visitor to whichever is closest? The answer is anycast, a technique that bends one of the internet's basic assumptions in a wonderfully useful way. Instead of one address meaning one machine, anycast lets one address mean "the nearest of many machines."

The problem anycast solves

The internet is enormous, and physics is stubborn. Data cannot travel faster than the speed of light, so a request from Australia to a server in Europe takes real, noticeable time no matter how fast the server is. For services that need to feel instant, and for services that must never go down, having a single server in a single location is a serious weakness. It is slow for distant users and it is a single point of failure.

The obvious fix is to put servers all over the world. But that raises a new question: how do you send each user to the right one automatically, without asking them to pick a "region" or remember a different address for each continent? Anycast answers that question at the level of the internet's own routing.

How anycast actually works

To understand anycast, it helps to know how the internet decides where packets go. Networks announce which IP address ranges they can reach using the Border Gateway Protocol, or BGP. Every router builds a map of the internet from these announcements and forwards each packet toward the best-known path for its destination.

Anycast works by having many separate servers, in many locations, all announce the same IP address range through BGP. From the routing system's point of view, that address now appears reachable from lots of places at once. When your packet sets off, each router simply forwards it along the shortest path it knows, which naturally leads to the nearest announcement. Two users on opposite sides of the world, typing the same IP address, quietly end up at two entirely different data centres, and neither of them has to do anything.

Anycast versus the other "casts"

Anycast is one of a small family of delivery methods, and comparing them makes its role clearer.

The vast majority of internet traffic is ordinary unicast, where an address maps to one machine. Anycast is the special case that keeps the simplicity of a single address while quietly spreading it across the globe.

Where anycast is used

You rely on anycast constantly without knowing it. The most important example is the Domain Name System. The root and top-level DNS servers, which the whole internet depends on, are served by anycast so that a lookup always reaches a nearby instance. Public DNS resolvers with memorable addresses use anycast to answer queries from close by, wherever you happen to be.

Content delivery networks lean on anycast heavily to route visitors to the nearest edge location. So do the large security providers that absorb attacks on behalf of websites. Anywhere the goals are "be close to everyone" and "never go dark," anycast tends to be part of the design.

Resilience and DDoS defence

Speed is only half the story. Anycast is also a powerful tool for resilience. If one location fails, whether from a power cut, a hardware fault, or maintenance, its BGP announcement simply disappears, and routers automatically steer users to the next-nearest instance. There is no manual failover and, for most users, no visible outage.

The same property is a natural defence against DDoS attacks. When attackers flood an anycast IP with junk traffic, that traffic is dispersed across every location announcing the address, rather than concentrated on one server. A flood that would flatten a single machine gets divided among many, and each site only has to weather its local share. This is why so much of the internet's DDoS protection is built on an anycast foundation.

The trade-offs

Anycast is not free of complications. Its great strength, that routing decides where you land, is also its main limitation. Internet routing can shift, and if it does mid-connection, a user could in principle be moved from one instance to another. For short, self-contained exchanges like a DNS query this is harmless, which is exactly why DNS was one of the earliest and best fits for anycast.

For long-lived, stateful connections, engineers have to work harder, ensuring each instance can pick up where another left off or keeping sessions pinned to one location. Running anycast also requires the ability to announce address space through BGP from multiple sites, which is the domain of larger operators rather than a typical small website.

Still, for the services that hold the internet together, DNS, content delivery, and attack mitigation, anycast is close to indispensable. It takes a single, simple idea, one address that means "the nearest of us," and turns it into a globe-spanning system that is both fast and hard to knock down. To keep exploring the protocols behind the scenes, browse the rest of the guides here at IP Animals.