Art of Codeartofcode
Lesson 3 of 15

4. DNS

1
The picture to keep in your head
2
4.1 Why do we need names at all?
3
4.2 What are the parts of a domain name?
4
4.3 What happens, step by step, when a name is looked up?
5
4.4 What kinds of DNS records are there?
6
4.5 What is a TTL, and why does DNS cache so much?
7
4.6 How can DNS spread users across servers or send them to the nearest one?
8
4.7 Why do DNS changes "take time to propagate"?
9
4.8 What happens when DNS fails?
10
Memory card and where the analogy breaks
11
Practice: five levels
12
Interview questions
13
Solutions
14
Sources
15
Code lab
4. DNS

4.2 What are the parts of a domain name?

3 min read+10 XP

In your phone's contacts: A full address reads from small to big: "flat 4, Green Apartments, Hyderabad, India". Each part narrows it down, and each level is managed by someone different.

In the system: A domain name is a list of labels separated by dots, and it reads right to left, from big to small.

anatomy of shop.example.com: subdomain, registered domain, TLD and root.

Figure: read a domain right to left. Each level only needs to know about the level just below it.

  • Root: the invisible dot at the very end (shop.example.com.). Every lookup starts here.
  • Top-level domain (TLD): .com, .org, .dev, or a country code like .in or .de.
  • Registered domain: example.com. You rent it yearly through a registrar. Whoever owns it controls everything below it.
  • Subdomain: shop, www, api. The owner creates as many as they like, for free, by adding records.

Limits worth knowing: each label can be up to 63 characters, and the whole name up to 253 characters in normal text form.

Why this shape matters: it lets the work be split up. The root only knows who runs each TLD. The .com servers only know who runs each .com domain. The bookstore's own name servers know everything about example.com. Nobody has to hold the whole internet's phone book.

Memory code: "Right to left, big to small."