DNS and CDNs

The Domain Name System (DNS) is the hierarchical, distributed naming system that translates human-readable domain names (e.g., example.com) into the IP addresses that computers use to route network traffic. Without DNS, users would need to memorize numeric addresses for every service they use; with it, the internet remains navigable while the underlying infrastructure (server IPs, hosting providers, even entire architectures) changes freely behind the scenes. A Content Delivery Network (CDN) is a geographically distributed network of proxy servers ('edge' or 'points of presence') that cache and serve content — static assets, and increasingly dynamic content too — from a location physically close to the requesting user, reducing latency and offloading traffic from origin servers. Together, DNS and CDNs form the first layer of a request's journey and are often the first two components sketched in a system design diagram for any internet-facing service.

DNS resolution is a multi-step lookup: a client's resolver (often provided by the ISP or a public resolver like 8.8.8.8) queries a hierarchy of nameservers — root servers, then top-level-domain (TLD) servers (e.g., .com), then the domain's authoritative nameserver — caching each answer according to its time-to-live (TTL) to avoid repeating the full lookup on every request. This caching hierarchy is what makes DNS scale to billions of daily queries: most lookups are served from a cache at the resolver or even the client's operating system, and only a cache miss triggers the full recursive lookup chain. DNS also supports load distribution: a domain can resolve to multiple IP addresses (round-robin DNS) or, more powerfully, use geo-DNS to return different IPs depending on the requester's location, directing users to the nearest available data center or CDN edge node.

A CDN works by placing edge servers in many geographic locations (often dozens to hundreds of points of presence globally) and routing each user's request to the nearest healthy edge, typically via DNS-based or Anycast-based routing. On a cache hit, the edge serves the content directly without ever contacting the origin server, cutting round-trip latency from potentially hundreds of milliseconds (cross-continent) to single-digit milliseconds (local edge), while also shielding the origin from traffic spikes — a property especially valuable during flash-crowd events. Modern CDNs extend beyond static assets (images, CSS, JS, video) to cache API responses and even execute logic at the edge (edge computing/edge functions), further reducing the work the origin must do per request.