What is a CDN? Content Delivery Networks Explained

A Content Delivery Network (CDN) is a geographically distributed network of servers that delivers web content from locations close to end users. CDNs reduce latency, improve load times, and absorb traffic spikes by caching content at the network edge — often within the same city as the user.

How CDNs Work

When you visit a website that uses a CDN, your request is routed to the nearest CDN server (called an edge node or Point of Presence) rather than the website's origin server, which might be thousands of miles away. The edge node either serves the content from its cache or fetches it from the origin and caches it for subsequent requests.

CDNs use several techniques to route users to the nearest edge:

• DNS-based routing — The CDN's DNS server responds with different IP addresses based on the user's location, directing them to a nearby edge node.
• BGP Anycast — The same IP address is announced via BGP from multiple locations worldwide. The internet's routing system naturally directs each user to the topologically nearest instance.

Anycast and BGP

Anycast is a key technology for CDNs. With anycast, a single IP address like 1.1.1.1 (Cloudflare's DNS) is announced from hundreds of locations simultaneously. When you look up this address in the looking glass, you may see multiple routes from different peers — each representing a different path to a different physical server, all sharing the same IP.

This is why an anycast address may show multiple BGP routes with different AS paths — the BGP routing system sees each announcement independently.

Major CDN Providers

The largest CDNs operate thousands of servers across hundreds of cities:

• Cloudflare (AS13335) — Operates in 300+ cities, peers at 300+ IXPs
• Akamai (AS20940) — One of the oldest CDNs, embedded within ISP networks worldwide
• Fastly (AS54113) — Edge cloud platform focused on programmability
• Amazon CloudFront (AS16509) — AWS's CDN service
• Google Global Cache — Google embeds cache servers directly inside ISP networks

CDNs and the Routing Table

CDN providers are among the most heavily peered networks on the internet. When you look up a CDN's ASN, you will see extensive peering relationships and often hundreds of announced prefixes. Their goal is to be as close to every user as possible — in network topology terms, within one or two AS hops of every ISP.

• AS13335 — Cloudflare's routes and peers
• AS20940 — Akamai's routes
• cloudflare.com — Resolve and see the BGP route