A content delivery network (CDN) is a geographically distributed network of servers that work together to deliver internet content faster and more reliably. Instead of all users accessing content from a single origin server, CDNs cache copies of static assets like images, videos, stylesheets, and JavaScript files on servers located closer to users around the globe.

Think of it like a retail chain. Instead of everyone traveling to one massive warehouse headquarters to buy products, there are local stores in every city stocked with popular items. You get what you need faster, and the central warehouse handles less traffic.

Some 46 million websites use CDNs(새 창) worldwide, including around 81% of the top 1 million websites(새 창). Major CDN providers include Akamai Technologies and the almost ubiquitous Cloudflare (over 24 million websites use Cloudflare(새 창) alone).

Why do CDNs exist?

Traditionally, websites were hosted on a single server in one physical location. Which works fine if all your visitors live near that server. But once users are spread across countries (or continents), problems appear:

  • Latency (slow loading times due to physical distance)
  • Bandwidth bottlenecks
  • Server overload during traffic spikes
  • Higher vulnerability to DDoS attacks(새 창)

CDNs solve this by distributing content across many servers worldwide. This means if you’re in New York, you’ll get content from a US-based sever, while visitors from London to the same website will get be served content from a European sever.

What are the benefits of CDNs?

CDNs improve:

  • Speed: Content travels a shorter physical distance
  • Scalability: Traffic spikes (e.g., during sales or viral events) don’t crash the site
  • Uptime: If one server fails, another takes over
  • Security: Most CDNs help protect against DDoS attacks

How do CDNs work?

A CDN consists of:

  1. An origin server. This is the “main” server where the website actually lives.
  2. Edge servers that store (“cache”) static content like images, CSS files, JavaScript, and videos from the origin server. Large CDNs (like Cloudflare) may run hundreds of these distributed edge servers all over the world.

When you visit a website that uses a CDN, the process works as follows:

  1. User request: When you visit a website, your browser sends a request for content.
  2. DNS resolution: The CDN’s DNS system determines which server is best positioned to serve you. Typically, this will be the one closest to you.
  3. Cache check: The edge server checks if it has the requested content cached.
  4. Delivery: If cached, the content is delivered immediately. If not, the edge server fetches it from the origin server, caches a copy for future requests, and delivers it to you.
  5. Dynamic content: For dynamic content (like personalized pages), CDNs can route requests to the origin server while still optimizing the connection path.

Crucially, all websites that use the same CDN will route their content though the CDN’s shared edge servers. This means hundreds of otherwise unrelated websites serving a similar geographic area may share the IP address of a CDN edge server for that area.

CDNs and the dangers of overblocking

To combat illegal streaming of its football matches, top Spanish football association LaLiga has obtained court orders requiring Spanish internet service providers to block thousands of IP addresses belonging to CDN provider Cloudflare. This is because it has connected these IP addresses to illegal streaming services.

Services affected by Cloudflare blocks in Spain

The problem is these IP addresses belong to Cloudflare’s edge servers, which are also used by thousands (potentially millions) of legitimate websites. This has left millions of people in Spain unable to reliably access some popular sites, including(새 창) GitHub, ChatGPT, government websites, and banking services.

Final thoughts on CDNs

Content delivery networks are now essential infrastructure for today’s web, quietly powering the fast, reliable online experience that you’ve come to expect. By distributing content across geographically dispersed edge servers, CDNs reduce latency, absorb traffic spikes, improve uptime, and add critical layers of security.

Much of their utility lies in their invisibility. Although used by over three-quarters of all websites on the internet, most of us never even think about them. But they’re there, working behind the scenes on roughly three-quarters of today’s websites, ensuring that content arrives quickly and reliably regardless of geography.

Recent situations where governments, courts and corporate interests have blocked internet services inevitably do significant collateral damage to the wider internet.