DNS (Domain Name System)

DNS translates human-readable domain names (google.com) into machine-readable IP addresses (142.250.190.46). It’s the “phonebook of the internet.”

DNS Hierarchy

Root (.) — 13 root server clusters (e.g., a.root-servers.net)
  └── TLD (.com, .org, .net) — managed by registries (Verisign for .com)
       └── Authoritative (example.com) — the domain owner's DNS server

Level	Role	Who runs it
Root	Knows where TLD servers are	ICANN (13 logical root clusters, hundreds of physical servers)
TLD	Knows where authoritative servers for each domain are	Registry operators
Authoritative	Knows the actual IP address for each domain	Domain owner (or their DNS provider)

How DNS Resolution Works

Recursive Query

Your browser asks the recursive resolver (usually your ISP’s or Cloudflare’s 1.1.1.1) to find the IP. The resolver does all the work.

Iterative Query

The resolver asks each level in turn:

1. Ask Root: “Who manages .com?” → Root points to TLD servers
2. Ask TLD (.com): “Who manages google.com?” → TLD points to Google’s authoritative server
3. Ask Authoritative (ns1.google.com): “What is the IP of google.com?” → Returns IP

DNS Record Types

Record	Purpose	Example
A	Maps domain → IPv4	google.com → 142.250.190.46
AAAA	Maps domain → IPv6	google.com → 2607:f8b0:...
CNAME	Maps domain → another domain (alias)	www.example.com → example.com
MX	Maps domain → mail server	@ → mail.example.com (with priority)
NS	Maps domain → name server	example.com → ns1.example.com
TXT	Arbitrary text (verification, SPF)	Used for email authentication

DNS Uses UDP (Usually)

DNS queries are small and fast. UDP avoids the TCP handshake overhead. If the answer doesn’t fit in a single UDP packet (>512 bytes), the resolver falls back to TCP (via truncation flag).