Chapter 9: Domain Names, DNS and Subdomains

9.2 How DNS resolution works

By Ravindra Bagale · Last updated: 25 September 2026

1. The browser checks its own cache, then the OS cache and the hosts file.
2. If not found, it asks the recursive resolver (your ISP, a public resolver like 8.8.8.8 / 1.1.1.1, or on EC2 the VPC resolver).
3. The resolver asks a root server → "ask the .com servers".
4. The .com TLD server → "the name servers for example.com are ns51.domaincontrol.com...". These NS entries come from what you set at the registrar.
5. The authoritative name server replies with the record, e.g. A 13.233.10.25, along with a TTL.
6. The resolver caches the answer for TTL seconds and gives it to the browser, which then connects to the IP and sends Host: example.com.

DNS normally uses UDP port 53 (TCP 53 for large answers and zone transfers). See also section 2.4 on opening a website step by step.