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Definition
A CDN (Content Delivery Network) is a geographically distributed network of servers that delivers web content to users from the server closest to them. CDNs reduce latency, speed up page loads, and protect against traffic spikes and DDoS attacks.
When a user in Tokyo requests an image from a website hosted in New York, the data must cross the Pacific Ocean — adding hundreds of milliseconds of latency. A CDN solves this by caching copies of that image on edge servers in Tokyo (and hundreds of other locations worldwide). The first request fetches from the origin server; subsequent requests are served from the nearest edge server, dramatically reducing load times.
CDNs are most effective for static assets: images, CSS, JavaScript, fonts, videos, and downloadable files. Modern CDNs also handle dynamic content through techniques like edge computing (running code at the edge), dynamic site acceleration (optimizing the route between edge and origin), and full-page caching with intelligent invalidation. Major CDN providers include Cloudflare, AWS CloudFront, Fastly, and Akamai.
CDNs absorb traffic spikes — a viral blog post or product launch that would overwhelm a single server is distributed across the CDN's global network. DDoS protection is built in, since attack traffic is spread across many edge nodes. CDNs also reduce bandwidth costs for origin servers and improve SEO, since Google uses page speed as a ranking factor. For any website with a global audience, a CDN is essential infrastructure.