What Is DNS?
The Domain Name System is the internet's address book — it's what lets you type youtube.com instead of a raw IP address like 142.250.185.78.
The basic idea
Every device connected to the internet has an IP address — a numerical label that identifies it on the network. Humans are bad at remembering long strings of numbers, so the Domain Name System was created to bridge the gap. DNS maps friendly domain names like github.com to the IP addresses that servers actually use to route traffic.
Think of it like a phonebook: you know a person's name, you look them up, and you get their number. With DNS, your browser knows the domain name, it looks it up, and it gets the IP address it needs to open a connection.
How a DNS lookup works
When you type a URL into your browser, a lookup happens in milliseconds:
- 1Browser cache: Your browser checks whether it already has the IP address stored locally from a recent visit.
- 2Operating system cache: If not cached in the browser, the OS checks its own DNS cache.
- 3Recursive resolver: Your ISP (or a public resolver like 1.1.1.1) receives the query and does the heavy lifting on your behalf.
- 4Root nameservers: The resolver asks the root nameservers which authoritative server is responsible for the top-level domain (e.g. .com).
- 5Authoritative nameserver: The resolver queries the authoritative DNS server for the domain, which returns the actual IP address.
- 6Connection: Your browser receives the IP address and opens a connection to the web server.
DNS record types
DNS isn't just one record — it's a set of different record types, each serving a different purpose:
AMaps a domain to an IPv4 address. The most common record type.AAAAMaps a domain to an IPv6 address.CNAMEAn alias that points one domain name to another (e.g. www → bare domain).MXSpecifies which servers handle email for the domain.NSIdentifies the authoritative nameservers for the domain.TXTHolds arbitrary text data, often used for domain verification and email authentication.What happens when DNS fails
DNS failures are a surprisingly common cause of websites appearing "down" even though the web server is perfectly healthy. If a DNS record is misconfigured, expired, or the DNS provider is experiencing an outage, browsers simply cannot resolve the domain to an IP address — and the site becomes unreachable.
The error you'll typically see in your browser is DNS_PROBE_FINISHED_NXDOMAIN (Chrome) or "Server Not Found" (Firefox). These errors mean the DNS lookup returned no result — either because the domain doesn't exist, the record has expired, or the resolver can't reach the authoritative nameserver.
DNS propagation and TTL
Every DNS record has a TTL (Time To Live) value — a number in seconds that tells resolvers how long to cache the result before looking it up again. A TTL of 3600 means the record is cached for one hour. When a website changes its IP address (e.g. moving to a new server), the old cached records may keep users pointing at the old IP until the TTL expires. This is why DNS changes can take minutes to hours to take effect globally — a process called DNS propagation.