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, 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 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. And even where caching is configured, each PoP
caches independently: a
hit ratio that looks fine globally can hide
permanently cold caches in remote regions, as
our 408,000-request measurement showed.
Tiered caching, an
origin shield and
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 to see the cache status per region, or the website speed test to see what a first vs. repeat visit costs from 8 locations. Both detect the major CDNs automatically.