DNS

There are millions of servers around the world, but when you type chatgpt.com into your browser, how does it know which server to connect to?
That is where DNS (Domain Name System) comes in.
Think of DNS as the internet's contacts app:
- A contacts app maps names to phone numbers.
- DNS maps domain names to IP addresses.

Example: pointing a domain to a website

Suppose you buy desisubtitles.com from GoDaddy or Porkbun and host your website on Vercel.
In your domain's DNS settings, you add an A record pointing to the IPv4 address of your Vercel server.
When someone enters desisubtitles.com, the browser starts a DNS lookup using a DNS resolver, such as Google or Cloudflare.

The DNS resolver asks a Root DNS Server for the .com nameservers.
The .com nameservers are managed by Verisign and direct the resolver to the authoritative nameservers for your domain.
The resolver asks the authoritative nameserver for the A record containing your server's IP address.
The resolver returns that IP address to the browser, and the browser connects to the server.

Imagine migrating your website from Vercel to EC2:
- Your EC2 instance has a new IP address.
- You update the A record in your DNS settings.
- Some users may still connect to the old IP address.
This happens because of DNS caching.
- DNS resolvers temporarily cache responses, so they do not need to repeat the full DNS lookup for every request.
- Every DNS record has a TTL (Time To Live), which specifies how long the record can be cached.
- Once the cached record expires, the resolver can fetch the updated IP address.

DNS translates domain names into IP addresses using resolvers, Root DNS Servers, and authoritative nameservers.
DNS resolvers return IP addresses, while caching and TTL control how long those answers can be reused.