The URL Journey: From Input to Page

This is one of the most common interview questions for software engineers. A complete answer spans multiple layers.

Step-by-Step Breakdown

1. URL Parsing

The browser parses the URL: https://www.google.com/search?q=hello

Component	Example
Protocol	https
Domain	www.google.com
Path	/search
Query	?q=hello

2. DNS Resolution

Browser checks caches: Browser → OS → Router → ISP resolver. If none have it cached, a recursive DNS query resolves www.google.com → 142.250.190.46.

3. TCP 3-Way Handshake

Browser opens a TCP connection to the server’s IP on port 443: SYN → SYN-ACK → ACK.

4. TLS Handshake (HTTPS)

If HTTPS, the client and server negotiate encryption:

• Client sends supported ciphers
• Server responds with certificate (public key)
• Client verifies certificate against trusted CAs
• Symmetric session key is exchanged
• Secure tunnel established

5. HTTP Request

Browser sends an HTTP GET request:

GET /search?q=hello HTTP/1.1
Host: www.google.com
User-Agent: Chrome/...
Accept: text/html

6. Server Processing

The server receives the request, processes it (lookup, query, computation), and generates a response.

7. HTTP Response

HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 45200

Followed by the HTML body.

8. Browser Rendering

1. Parse HTML → DOM tree
2. Parse CSS → CSSOM (CSS Object Model)
3. Combine → Render tree
4. Layout → compute positions
5. Paint → pixels to screen
6. JavaScript executes (may trigger re-layouts)

9. Additional Requests

The browser parses the HTML and makes additional requests for CSS files, JavaScript, images, fonts, etc. Each may trigger new TCP connections (or reuse existing ones via HTTP/2 multiplexing).