CDN: How It Works and Why You Need It

What Is a CDN

A Content Delivery Network (CDN) is a distributed network of servers located around the world. Instead of loading content from a single origin server, users receive it from the nearest CDN edge server. This shortens the distance data travels and consequently reduces latency.

If your server is in New York and a user is in Tokyo, data travels approximately 11,000 km. With a CDN, the Tokyo user gets content from an edge server in Japan — distance drops to hundreds of kilometers.

How CDN Accelerates Delivery

Edge Servers and PoPs

A CDN consists of Points of Presence (PoPs) — data centers at various locations worldwide. Each PoP contains edge servers storing cached copies of content. Major CDNs have 200-300+ PoPs.

Anycast Routing

CDNs use anycast — a technology where a single IP address is announced from multiple locations. Network infrastructure automatically routes requests to the nearest server based on BGP metrics.

Caching

Edge servers cache content according to HTTP headers (Cache-Control, s-maxage). On the first request, content is fetched from the origin server (cache miss). Subsequent requests are served from cache (cache hit).

Connection Optimization

• TLS termination — TLS handshake happens at the edge server, closer to the user
• HTTP/2 and HTTP/3 — modern protocol support without origin configuration
• TCP optimization — pre-established connections between edge and origin
• Compression (Brotli/gzip) — automatic compression at the edge

What to Cache Through CDN

Static Resources (Must)

• CSS, JavaScript files
• Images (JPEG, PNG, WebP, AVIF, SVG)
• Fonts (WOFF2, WOFF)
• Video and audio files
• PDFs and other documents

Dynamic Content (Optional)

• HTML pages with short TTLs
• API документацию responses with public data
• Search results (cached by query string)

Don't Cache

• Personalized content (dashboards, shopping carts)
• Responses with Set-Cookie
• POST/PUT/DELETE requests
• Authenticated pages

CDN and Security

DDoS Protection

A CDN absorbs DDoS attacks thanks to the massive bandwidth of its distributed network. An attack that would take down a single server is spread across hundreds of edge servers.

WAF

Many CDNs include a Web Application Firewall that filters malicious requests before they reach the origin server.

Bot Protection

Detection and blocking of bots, scrapers, and automated attacks.

SSL/TLS

CDNs provide automatic SSL certificates and support for the latest TLS versions.

When You Don't Need a CDN

• Local audience — if all users are in one city/region, a single server may suffice
• Low traffic — CDN overhead won't be justified
• Fully dynamic content — if there's nothing to cache, CDN provides minimal benefit
• Regulatory restrictions — if data can't leave a specific jurisdiction

How to Verify CDN Operation

Use the Enterno.io HTTP Checker to analyze response headers. CDNs typically add their own headers: CF-Cache-Status (Cloudflare), X-Cache (AWS CloudFront), X-CDN (others). Also check response time with the Ping Test and Speed Test.

CDN Monitoring

A CDN adds an abstraction layer that needs monitoring:

• Cache hit ratio — should be above 85-90%
• Edge server response times
• Origin pulls — frequency of requests to the origin server
• 5xx errors on edge and origin

Set up uptime monitoring to track availability through the CDN.

Summary

A CDN is a fundamental infrastructure component for websites with global audiences. It speeds up loading, reduces origin server load, protects against DDoS, and improves SEO through load times. Configure CDN for static resources and monitor cache hit ratio for maximum effectiveness.