---
title: Anycast Explained: one IP, served from everywhere
description: Anycast is the clever routing trick behind fast CDNs and resilient DNS: one IP address answered by the nearest of many servers. Here's how it works and why it matters.
canonical: https://watchfor.io/blog/anycast-explained
---

# Anycast Explained: one IP, served from everywhere

2026-01-05 · WatchFor Team · Networking · networking, performance, basics

Anycast is the clever routing trick behind fast CDNs and resilient DNS: one IP address answered by the nearest of many servers. Here's how it works and why it matters.

How can a single IP address be served from dozens of cities at once, always routing you to the nearest one? It sounds impossible, but it's the everyday magic behind [CDNs](/blog/what-is-a-cdn), big DNS resolvers (like `1.1.1.1`), and DDoS-resilient infrastructure. The trick is called **anycast**.

## The "casts" — a quick map

Networking has a few ways to address traffic:

| Type | Means | Like… |
| --- | --- | --- |
| **Unicast** | One sender → one specific destination | A phone call to one person |
| **Broadcast** | One sender → everyone on the network | A PA announcement |
| **Anycast** | One address → the *nearest* of many | Calling a hotline that routes to your local branch |

Most internet traffic is **unicast** (one specific server). **Anycast** is the interesting one: the *same* IP address is announced from **many locations**, and the network delivers your request to whichever is closest.

## How anycast works

The magic happens in the internet's routing layer ([BGP](/blog/what-is-bgp)):

1. The same IP address is **advertised from many locations** around the world.
2. When you send a request to that IP, the network's routing picks the **nearest** advertisement (by network distance).
3. Your request lands at the closest server — automatically, with no special setup on your end.

You just connect to one IP; the internet quietly routes you to the best instance.

## Why it's so useful

| Benefit | Why it matters |
| --- | --- |
| **Lower [latency](/blog/what-is-latency)** | You reach the nearest server, not a distant one |
| **Resilience** | If one location fails, traffic shifts to the next nearest |
| **Load distribution** | Requests naturally spread across locations |
| **DDoS absorption** | An attack is split across many locations, not one target |

That last point is big: anycast spreads a flood of attack traffic across the whole network instead of concentrating it on a single server — a key reason large [DNS](/blog/how-dns-works) and CDN providers use it.

## Where you've seen it

- **[CDNs](/blog/what-is-a-cdn)** use anycast so you hit the nearest edge automatically.
- **Public DNS resolvers** (the famous `8.8.8.8`, `1.1.1.1`) are anycast — the same IP answers from hundreds of locations worldwide.
- **DDoS protection** services rely on it to absorb and spread attacks.

> **The one-line idea:** anycast = **one address, many servers, nearest wins.** You connect once; the routing layer hands you off to the closest, healthiest instance.

## A monitoring note

Because an anycast IP is answered by *different* physical servers depending on where you are, monitoring it from [multiple locations](/blog/global-monitoring-locations) is the only way to see the whole picture — one region might hit a healthy node while another hits a struggling one, all behind the same address.

## The bottom line

| | In one line |
| --- | --- |
| **What** | One IP advertised from many locations; nearest answers. |
| **How** | The internet's routing (BGP) picks the closest instance. |
| **Why** | Lower latency, resilience, load spreading, DDoS defence. |
| **Seen in** | CDNs, public DNS resolvers, DDoS protection. |

Anycast is one of those quietly brilliant internet tricks: the same address, served from everywhere, always picking the nearest. It's why a CDN feels local and why big DNS resolvers shrug off attacks.

Related: [What is a CDN?](/blog/what-is-a-cdn), [What is BGP?](/blog/what-is-bgp), [how DNS works](/blog/how-dns-works).

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Canonical page: https://watchfor.io/blog/anycast-explained · All articles: https://watchfor.io/blog/all.md · Site guide: https://watchfor.io/llms.txt