Glossary

DNS

DNS (Domain Name System) is the internet's phonebook: it translates domain names (example.com) into IP addresses. A browser does not know where a site is physically located — DNS provides the address.

Record types

A — domain → IPv4 (93.184.216.34)
AAAA — domain → IPv6
CNAME — alias; one domain points to another domain
MX — mail servers for the domain
TXT — arbitrary text: SPF, DKIM, ownership verification

How resolution works

The browser queries the local DNS resolver → the ISP's recursive resolver → the root server → the TLD server (.com) → the domain's authoritative server → IP address. The whole journey takes tens of milliseconds and is cached for the TTL.

TTL

Time To Live — how long a record is cached. Before changing an A record, lower the TTL to 300 seconds, make the change, then restore it. This minimises downtime during a migration.