Info & Theory
The Domain Name System (DNS) is the internet's phone
book: it maps human-readable names like
www.example.com to IP addresses such as
93.184.216.34. No single server holds every name —
the database is distributed in a hierarchy.
A recursive lookup
- Your device's stub resolver asks a recursive resolver (often your ISP or 1.1.1.1).
-
The resolver asks a root server, which points to the
.comTLD server. -
The TLD server points to the authoritative server for
example.com, which returns the IP.
Caching with TTL
Each answer carries a Time To Live (TTL). The resolver caches it, so repeat queries are answered instantly without walking the hierarchy again. When the TTL expires the entry is removed and the next query is a full lookup.
Why it matters
Caching is what keeps DNS fast and the root servers from being overwhelmed. A short TTL lets records change quickly (useful for failover); a long TTL maximises cache hits and reduces load.
Record types & iterative vs recursive
DNS carries many record types — A,
AAAA, MX, CNAME,
TXT. The resolver does iterative queries to
each server, while it gives your device a single
recursive answer.
Frequently asked questions
What is DNS?
DNS, the Domain Name System, is the internet's directory. It translates human-readable domain names like www.example.com into the numeric IP addresses computers use to connect.
What is a recursive DNS lookup?
A recursive lookup is when a resolver does all the work on your behalf: it queries the root, TLD and authoritative servers in turn and returns a single final answer to your device.
What is the order of DNS servers queried?
A stub resolver asks a recursive resolver, which asks a root server, then the relevant TLD server, then the authoritative server for the domain, which returns the IP address.
What is DNS caching?
Caching stores recent answers so repeat queries can be answered instantly without walking the server hierarchy again. It makes DNS fast and reduces load on upstream servers.
What is a TTL in DNS?
TTL, or Time To Live, is how long a cached record stays valid. After it expires the entry is removed and the next query triggers a fresh full lookup.
Why does the first lookup take longer?
The first lookup is a cache miss, so the resolver must contact the root, TLD and authoritative servers. Once cached, repeat queries for the same name are near-instant cache hits.
What is a root DNS server?
A root server sits at the top of the hierarchy. It does not know individual addresses but points the resolver to the correct top-level-domain server, such as the one for .com.
What is an authoritative DNS server?
An authoritative server holds the actual records for a domain. It is the final source of truth that returns the IP address for a name like example.com.
Should I use a short or long TTL?
A short TTL lets records change quickly, which helps failover but increases query load. A long TTL maximises cache hits and reduces load but makes changes propagate slowly.
What record types does DNS use?
Common types include A and AAAA for IPv4 and IPv6 addresses, MX for mail, CNAME for aliases and TXT for arbitrary text such as verification records.