What Is Anycast? Why DNS and CDNs Love It

Imagine ordering a pizza from a chain restaurant. You call their single phone number, but the call gets routed to the location closest to you. That's essentially how Anycast works in networking — one IP address that exists in multiple locations, with traffic automatically routed to the nearest or best instance.

The Anycast Mental Model

In traditional networking (called Unicast), each IP address belongs to exactly one device in one location. If that device goes down or becomes unreachable, the service stops working. Anycast breaks this rule by allowing the same IP address to be announced from multiple locations simultaneously.

When you send traffic to an Anycast IP address, the internet's routing system automatically delivers it to the "closest" instance based on network topology and routing policies. This happens transparently — you don't need to know or care which specific server answered your request.

Why DNS Loves Anycast

DNS is the perfect use case for Anycast because DNS queries are typically small, stateless requests. When you type google.com into your browser, your computer needs to resolve that domain to an IP address quickly.

Major DNS providers like Cloudflare (1.1.1.1) and Google (8.8.8.8) use Anycast extensively. That single 1.1.1.1 IP address is actually hosted on hundreds of servers worldwide. When you query it:

• Your request goes to the geographically or network-topologically closest server
• Response time is minimized because the server is nearby
• If one server fails, traffic automatically routes to the next best option
• No configuration changes are needed on your end

CDNs and the Latency Game

Content Delivery Networks (CDNs) face a similar challenge — how do you serve content quickly to users worldwide? Anycast provides an elegant solution.

Instead of giving users a complex system to choose the best server, CDNs announce the same IP addresses from multiple edge locations. When someone in Tokyo requests content, they automatically connect to the Tokyo edge server. Someone in London connects to the London server, all using identical IP addresses.

This approach delivers two key benefits:

• Reduced latency: Traffic takes the shortest network path to reach a server
• Automatic failover: If a server location goes offline, traffic seamlessly reroutes to the next best location

A Simple Traceroute Experiment

You can observe Anycast in action with a simple experiment. Try running traceroutes to known Anycast addresses from different locations:

traceroute 1.1.1.1
traceroute 8.8.8.8

If you have friends or colleagues in different cities or countries, ask them to run the same commands. You'll likely see different paths and different final hop locations, even though you're all targeting the same IP addresses. This demonstrates Anycast routing traffic to geographically distributed servers.

You might also notice that the final few hops have different hostnames or geographic indicators, revealing which specific data center handled your request.

The Technical Foundation

Anycast works through BGP (Border Gateway Protocol) announcements. Multiple servers announce the same IP prefix from different locations. Internet routers see these announcements and choose the "best" path based on their routing tables, typically preferring shorter AS-paths or lower-cost routes.

This creates automatic load balancing and redundancy without requiring complex client-side logic or DNS-based geographic routing systems.

Limitations to Consider

Anycast isn't perfect for every use case. It works best for stateless protocols like DNS and HTTP requests to static content. For stateful connections like long-running TCP sessions, Anycast can cause problems if traffic switches between servers mid-session.

Additionally, Anycast routing decisions happen at the network level and may not always reflect true geographic proximity or server performance.

What's Next

Now that you understand Anycast fundamentals, we'll explore how BGP makes this magic possible and dive into specific implementation strategies that major providers use to optimize their Anycast networks.