What happens when you type a URL into the browser

When you type a URL into your browser and press Enter, a series of steps take place to load the requested web page. Here's a simplified breakdown of the process:

1. URL Parsing:

   • The browser parses the URL to determine the protocol (e.g., HTTP or HTTPS), the domain name, and the path to the resource.

2. DNS Lookup:

   • The browser checks its cache for the IP address of the domain. If it's not found, it queries a DNS (Domain Name System) server to translate the domain name into an IP address.

3. TCP/IP Connection:

   • The browser establishes a TCP (Transmission Control Protocol) connection with the server using the IP address obtained from the DNS lookup. This involves a three-way handshake:
     1. The browser sends a SYN (synchronize) packet to the server.
     2. The server responds with a SYN-ACK (synchronize-acknowledge) packet.
     3. The browser sends an ACK (acknowledge) packet back to the server.

4. HTTP/HTTPS Request:

   • If the URL uses HTTPS, the browser initiates a TLS (Transport Layer Security) handshake to establish a secure connection. This involves certificate verification and encryption key exchange.
   • The browser then sends an HTTP or HTTPS request to the server, specifying the desired resource (e.g., a web page or file).

5. Server Processing:

   • The server receives the request, processes it, and locates the requested resource. It may involve fetching data from a database or executing server-side scripts.

6. HTTP/HTTPS Response:

   • The server sends an HTTP or HTTPS response back to the browser. This response includes status codes (e.g., 200 OK, 404 Not Found) and the requested resource (e.g., HTML, CSS, JavaScript, images).

7. Rendering:

   • The browser receives the response and begins rendering the web page. This involves several steps:
     • Parsing the HTML to construct the DOM (Document Object Model) tree.
     • Parsing the CSS to apply styles and construct the CSSOM (CSS Object Model) tree.
     • Parsing and executing JavaScript.
     • Combining the DOM and CSSOM trees to create the render tree.
     • Painting the render tree onto the screen.

8. Resource Loading:

   • As the browser parses the HTML, it may encounter additional resources (e.g., images, stylesheets, scripts) that need to be loaded. It sends additional HTTP/HTTPS requests to fetch these resources and updates the web page as they are received.

9. Interaction and Updates:

   • Once the web page is loaded, the browser continues to handle user interactions (e.g., clicks, form submissions) and updates the page dynamically as needed (e.g., via AJAX requests).

This entire process happens very quickly, typically within a fraction of a second, allowing users to access web pages almost instantaneously.

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