DNS

The internet's phone book — it translates human-readable domain names like 'example.com' into the IP addresses computers actually use.

Record types: A (domain → IPv4), AAAA (domain → IPv6), CNAME (alias to another domain), MX (mail servers), TXT (verification, SPF, DKIM). TTL controls how long resolvers cache records.

Recursive resolver queries root → TLD nameserver → authoritative nameserver. DNSSEC adds cryptographic signatures to prevent cache poisoning. Anycast routes DNS queries to the nearest datacenter.