DNS & CDN

DNS turns domain names into IP addresses. CDN serves static content from servers close to the user.

DNS Resolution

sequenceDiagram
    participant User as 👤 Browser
    participant DNS as 📖 DNS Resolver
    participant Root as 🌐 Root NS
    participant TLD as 🔤 .com NS
    participant Auth as 🏢 example.com NS

    User->>DNS: What's the IP for example.com?
    DNS->>Root: Where's .com?
    Root->>DNS: Ask .com name server
    DNS->>TLD: Where's example.com?
    TLD->>DNS: Ask example.com's name server (e.g., AWS Route 53)
    DNS->>Auth: What's the IP for example.com?
    Auth->>DNS: It's 192.0.2.1
    DNS->>User: It's 192.0.2.1
    User->>Auth: HTTP GET /

Key record types:

A — maps domain → IPv4 address
AAAA — maps domain → IPv6 address
CNAME — maps domain → another domain (alias)
MX — mail server records

CDN (Content Delivery Network)

A CDN caches static files (images, CSS, JS, videos) at edge servers around the world.

flowchart LR
    User1["🇺🇸 User NY"] --> Edge1["CDN Edge<br/>🇺🇸 US East"]
    User2["🇩🇪 User Berlin"] --> Edge2["CDN Edge<br/>🇪🇺 EU Central"]
    User3["🇦🇺 User Sydney"] --> Edge3["CDN Edge<br/>🇦🇺 Australia"]
    Edge1 --> Origin["Origin Server"]
    Edge2 --> Origin
    Edge3 --> Origin

    style User1 fill:#7c3aed,color:#fff
    style User2 fill:#4f46e5,color:#fff
    style User3 fill:#6366f1,color:#fff
    style Edge1 fill:#059669,color:#fff
    style Edge2 fill:#059669,color:#fff
    style Edge3 fill:#059669,color:#fff
    style Origin fill:#dc2626,color:#fff

How a CDN serves a request:

User in Australia requests an image
DNS resolves to the nearest CDN edge (Australia, not US)
If the edge has the image cached → serves instantly (cache hit)
If not → edge fetches from origin, caches it, serves it (cache miss)

Why CDNs Matter

Metric	Without CDN	With CDN
Latency (AU → US)	~150-200 ms	~10-20 ms
Origin server load	High (all requests hit origin)	Low (only cache misses)
Bandwidth costs	High (full bandwidth on origin)	Lower (CDN absorbs traffic)
DDoS protection	❌	✅ Many CDNs absorb attacks

Trade-offs

CDNs serve cached content — not suitable for dynamic personalized data.
Cache invalidation is hard (users might see stale content).
CDN cost depends on bandwidth — can be significant for large files.

In Simple Words

DNS = the phonebook of the internet. It tells your browser where to find a website.
CDN = copies of your files stored in many locations around the world.
DNS + CDN = users get content from the nearest server → much faster load times.