DNS | Learn IT Fundamentals with Teacher Pochi

Key points

DNS is the "name resolution" mechanism that translates domain names into IP addresses.
Behind the scenes, invisible to the browser, multiple DNS servers and caches work together to find the answer.
If DNS goes down, you can't reach a site even if you know its domain.

What is DNS?

DNS (Domain Name System) is a mechanism that translates human-readable domain names into the IP addresses computers use for communication.

Domain
example.com

Translated by DNS

IP address
203.0.113.1

🔑 Everyday exampleIt's similar to typing an address into a car navigation system, which converts it into actual location data (latitude and longitude) to guide you. DNS's job is to translate the "address" — the domain — into "location data" a computer can understand.

How name resolution works

When a browser accesses a domain, multiple servers work together behind the scenes to track down the IP address.

① Browser requests
example.com

② Queries a
DNS server

③ IP address
is returned

④ Connects to
the server

In reality, it's not just a single DNS server — servers with different roles, such as "cache servers" and "authoritative DNS servers," work together hierarchically to find the answer.

Types of DNS records

DNS servers store "which domain corresponds to which piece of information" as "records." Some representative record types include the following.

A record Domain → IP address

CNAME record Domain → another domain name

MX record Domain → mail server

The role of DNS caching

Redoing every lookup from scratch each time would take too long, so once a domain-to-IP-address mapping has been looked up, it's stored as a "cache" for a set period and reused.

1st time:
query DNS

Save the result

From the 2nd time on:
reuse the saved result

🔑 Everyday exampleOnce you've memorized a friend's phone number, you can recall it from memory next time without checking the phone book. DNS caching works the same way — by remembering a looked-up result for a while, it saves the trouble of querying again.

What happens when DNS has a problem

If a DNS server goes down or a setting is misconfigured, the correct IP address can't be obtained, and the site becomes unreachable.

NormalThe domain returns the correct IP address

Resolution failureDisplays "This site can't be reached"

Propagation pendingDNS changes can take time to spread worldwide

Summary

DNS is the name resolution mechanism that translates domain names into IP addresses, powered by multiple servers and caches working together. When DNS has a problem, a site becomes unreachable even if the domain itself is correct — making it one of the essential things to check when responding to an infrastructure incident.