DNS

Computers route traffic using numeric IP addresses, but humans remember names. DNS (the Domain Name System) is the web's phone book — it translates example.com into 93.184.216.34. Every page load starts with a DNS lookup.

The Phone Book of the Internet

When you visit example.com, your browser first needs its IP address. DNS provides that lookup:

example.com  ──DNS──▶  93.184.216.34
(what humans type)     (what routers need)

Without DNS, you'd have to memorize IP addresses for every site. DNS also adds flexibility: a site can change servers (new IP) without changing its domain name.

How a Lookup Works

DNS is a hierarchy of servers, each responsible for part of the name. A full resolution walks down that tree:

1. Recursive resolver   your ISP/router's DNS — does the legwork
2. Root servers         "for .com, ask the .com TLD servers"
3. TLD servers (.com)   "for example.com, ask its authoritative server"
4. Authoritative server "example.com is 93.184.216.34"  ← the real answer

The recursive resolver does this walking on your behalf and hands back the final IP. You ask once; it may make several queries behind the scenes.

Common Record Types

A domain's DNS settings are a set of records, each mapping a name to something:

CNAME is an alias:  www.example.com ──▶ example.com ──▶ (A record) ──▶ IP

Caching and TTL

DNS lookups are cached at every level — your browser, OS, resolver — so the full walk doesn't happen every time. Each record carries a TTL (time to live) saying how long it may be cached:

TTL 3600  →  cache this answer for 1 hour before asking again

This is why DNS changes aren't instant: after you update a record, caches around the world keep serving the old value until their TTL expires — "DNS propagation."

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