DNS
also: Domain Name System
The internet’s phone book — it translates human domain names into machine IP addresses.
DNS (Domain Name System) is the distributed directory that turns a human-readable name like example.com into the IP address a machine can actually connect to. It is hierarchical (root, top-level domain, authoritative servers) and heavily cached at every layer, from your OS resolver to recursive resolvers run by ISPs.
Worked example: typing a URL triggers a DNS lookup that walks from the root down to the domain’s authoritative server, returns an IP, and caches it for the record’s TTL so the next visit skips the walk. Gotcha: caching is the double-edged sword — a low TTL lets you fail over quickly but multiplies lookups, while a high TTL is efficient but means a changed record can take hours to propagate, so DNS changes are never truly instant.