Definition
A CDN (content delivery network) is a network of servers in many locations that delivers your site's files from the one nearest each visitor.
CDN explained
Without a CDN, every visitor's request travels to your origin server, wherever it is. With a CDN, static files such as images, stylesheets and scripts, and often whole cached pages, are served from nearby edge servers, which shortens the distance and the time. CDNs also absorb traffic spikes and often provide security features such as firewalls and bot protection.
For SEO, a CDN usually improves Time to First Byte and Largest Contentful Paint for visitors far from the origin, and makes sites more resilient. It also introduces risks to check. Bot-protection and firewall rules at the CDN are one of the most common reasons AI crawlers and sometimes search engines are blocked, and caching rules can serve stale pages or the wrong version after changes.
After setting up or changing a CDN, test that important crawlers receive normal responses, that redirects and canonical URLs still behave, and that cache clearing works when content updates.
Example
Your brand serves customers in the US, UK and Australia from one server in Europe. Putting the site behind a CDN serves cached pages from nearby locations and cuts load times abroad. A follow-up check makes sure its bot rules still let Googlebot and OAI-SearchBot through.
Why it matters
A CDN is one of the most effective speed improvements for global audiences, but its security settings need checking so crawlers aren't blocked.
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Related terms
- TTFB (Time to First Byte)TTFB (Time to First Byte) is the time from requesting a page to receiving the first byte of the response; web.dev suggests 0.8 seconds or less.
- LCP (Largest Contentful Paint)LCP (Largest Contentful Paint) measures how long the largest image, video or text block in view takes to render; 2.5 seconds or less is good.
- AI crawlerAn AI crawler is an automated bot run by an AI company that fetches web pages to train models, to build a search index, or to answer a user's question.
- CachingCaching is storing copies of files or whole pages so later requests can be served quickly, without rebuilding or re-downloading them.