---
title: "What is a CDN? Content delivery networks explained — warmup.rocks Glossary"
description: "A CDN caches content at locations close to users and shields your origin. What CDNs actually do, why adding one doesn't mean your site is cached, and how per-PoP caching changes the picture."
canonical: https://warmup.rocks/glossary/cdn
---

# CDN (content delivery network)

A **CDN** is a network of servers distributed across many locations that sits between your visitors and your origin server. It terminates connections close to the user, caches responses at each [point of presence](https://warmup.rocks/glossary/point-of-presence), and only forwards to the origin what it can't answer itself. The result: shorter round trips, less origin load, and a buffer against traffic spikes.

## What a CDN actually does

-   **Caching** — stores responses at the edge and serves repeats without an origin round trip. This is the part that moves [TTFB](https://warmup.rocks/glossary/ttfb) from hundreds of milliseconds to single digits.
-   **Connection termination** — DNS, TCP and TLS handshakes happen at the nearby PoP instead of across an ocean, which speeds up even uncacheable requests.
-   **Routing** — anycast or DNS-based steering sends each user to a close, healthy location; some CDNs also optimize the path back to the origin (e.g. Cloudflare Argo).
-   **Protection** — DDoS absorption, WAF, bot filtering happen before traffic reaches you.

## The misconception that costs the most performance

Putting a CDN in front of a site does **not** mean the site is cached. Most CDNs cache static assets by default but pass HTML straight through — on Cloudflare that shows up as [`cf-cache-status: DYNAMIC`](https://warmup.rocks/blog/cf-cache-status). And even where caching is configured, each PoP [caches independently](https://warmup.rocks/glossary/point-of-presence): a [hit ratio](https://warmup.rocks/glossary/cache-hit-ratio) that looks fine globally can hide permanently cold caches in remote regions, as [our 408,000-request measurement](https://warmup.rocks/blog/cdn-cache-miss-data) showed. [Tiered caching](https://warmup.rocks/glossary/tiered-cache), an [origin shield](https://warmup.rocks/glossary/origin-shield) and [cache warming](https://warmup.rocks/blog/what-is-cache-warming) are the levers that close that gap.

## See what your CDN is doing right now

Run any URL through the [multi-location cache checker](https://warmup.rocks/cache-checker) to see the cache status per region, or the [website speed test](https://warmup.rocks/website-speed-test) to see what a first vs. repeat visit costs from 8 locations. Both detect the major CDNs automatically.