How DNS resolves a domain name

Caches are checked first; the full lookup walks down the name from right to left.

Model answer. DNS (Domain Name System) is a distributed, hierarchical database that maps names to records such as IP addresses.

Lookup sequence

1. Local caches: the browser checks its own DNS cache, then the operating system's resolver cache (and the hosts file).
2. Recursive resolver: if not cached, the OS asks a configured recursive resolver — usually the ISP's, or a public one like 8.8.8.8 or 1.1.1.1. If the resolver has the answer cached, it replies immediately.
3. Root servers: otherwise the resolver asks a root server, which does not know the address but replies with a referral to the servers for the .com top-level domain.
4. TLD servers: the .com servers reply with a referral to the authoritative name servers for example.com.
5. Authoritative server: this server holds the zone's records and returns the answer, e.g. an A record (IPv4) or AAAA record (IPv6) for www.example.com.
6. The resolver caches the answer for its TTL (time-to-live) and returns it to the client, which also caches it.

Recursive vs iterative

• The client's query to the resolver is recursive: "give me the final answer".
• The resolver's queries to root, TLD and authoritative servers are iterative: each server returns the best referral it has, and the resolver does the next step itself.

Other useful facts

• DNS mainly uses UDP port 53 for speed; it falls back to TCP port 53 for responses too large for UDP and for zone transfers. Encrypted variants include DNS over HTTPS (DoH) and DNS over TLS (DoT).
• Common record types: A, AAAA, CNAME (alias), MX (mail server), NS (name server), TXT (verification, SPF).

Common follow-up: "Why do DNS changes take time to 'propagate'?" Nothing is pushed to the world — resolvers keep serving cached records until their TTL expires. Lowering the TTL well before a planned migration makes the switch faster.