What is DNS?

• The Domain Name System translates human-friendly hostnames into machine IP addresses.
• Example: www.google.com → 172.217.18.36

• DNS is a core component of the Internet.

• Uses a hierarchical naming structure.

• .com

• example.com

• www.example.com

• api.example.com

Key DNS Terminology

• Domain Registrar – Service that manages domain name registrations (e.g., Route 53, GoDaddy).

• DNS Records – Mappings like A, AAAA, CNAME, NS.

• Zone File – File containing all DNS records for a domain.

• Name Server – Resolves DNS queries (Authoritative or Non-Authoritative).

• Top Level Domain (TLD) – .com, .us, .org, etc.

• Second Level Domain (SLD) – The domain name before the TLD (e.g., amazon.com).

DNS Structure Example

• Protocol: http

• Subdomain: api.www

• SLD: example

• TLD: .com

• Root: the invisible . at the end

• FQDN: api.www.example.com.

How DNS Works

1. Browser Lookup – Checks local DNS cache (TTL-based). If missing, queries the Local DNS Server.

2. Local DNS Server – Usually managed by ISP or company; starts a recursive query if no cached record.

3. Root DNS Server – Returns the name servers for the relevant TLD (e.g., .com). Managed by ICANN.

4. TLD DNS Server – Returns the name servers for the SLD (e.g., example.com). Managed by IANA.

5. Authoritative SLD DNS Server – Returns the IP address for the requested hostname.

6. Final Response – Local DNS caches the IP and returns it to the browser, which connects directly to the web server.

• Each level of the hierarchy narrows the search.

• DNS caching speeds up lookups and reduces load.

• TTL controls how long a record is cached before it expires.