What happens when I type in a URL?

When you type a URL (Uniform Resource Locator) into your web browser and press Enter, a sophisticated sequence of events unfolds, allowing your computer to locate and display the requested web page. This process involves your browser, the internet's naming system, servers, and various protocols working in harmony.

The Journey of a URL Request

The moment you hit Enter, your browser initiates a multi-step journey to fetch the desired content.

1. URL Parsing and Initial Checks

Your web browser first parses the URL to understand its components: the protocol (like http:// or https://), the domain name (e.g., www.google.com), the port (often implied, like 80 for HTTP or 443 for HTTPS), and the path to the specific resource.

• Protocol: Determines how data is exchanged (e.g., secure HTTPS vs. standard HTTP).
• Domain Name: The human-readable address of the website.
• Path: Specifies the exact page or file on the server.

The browser then performs a few quick checks:

• Browser Cache: Has it recently accessed this URL? If so, and the content is still fresh, it might serve the page directly from its local cache, significantly speeding up load times.
• HSTS Preload List (for HTTPS): If the URL uses http:// but the domain is on the browser's HTTP Strict Transport Security (HSTS) preload list, the browser will automatically upgrade the connection to https:// to ensure security, preventing insecure connections.

2. Domain Name Resolution (DNS Lookup)

Since computers communicate using numerical IP addresses, not domain names, your browser needs to translate the domain name into an IP address.

• Local Checks: Your computer first checks its own local DNS cache and then asks your operating system (OS). If not found, your OS might check a local hosts file.
• Router Query: Next, the request goes to your router, which also maintains a DNS cache.
• DNS Server Request: If the IP address isn't found locally, your computer sends a request to the domain name system (DNS) server which serves as an address book for all domain names. This DNS server is typically provided by your Internet Service Provider (ISP).
• Hierarchical Search: If your ISP's DNS server doesn't have the record, it queries other DNS servers (root servers, top-level domain servers, and authoritative name servers) in a hierarchical fashion until it finds the correct IP address for the domain.
• IP Address Return: This then sends back the exact IP address of the server which the URL points to. For example, www.google.com might resolve to 142.250.190.132.

3. Establishing a Connection (TCP/IP Handshake)

Knowing this IP, your computer then establishes a connection with the server through the IP address. This connection is typically built using the Transmission Control Protocol (TCP), a reliable connection-oriented protocol that ensures data is sent and received correctly.

• Three-Way Handshake: TCP establishes a connection using a "three-way handshake":
  1. Your computer sends a SYN (synchronize) packet to the server.
  2. The server responds with a SYN-ACK (synchronize-acknowledge) packet.
  3. Your computer sends an ACK (acknowledge) packet, confirming the connection.
• Security (TLS/SSL Handshake for HTTPS): If the URL uses https://, an additional TLS/SSL handshake occurs before any application data is sent. This handshake encrypts the connection, ensuring that all subsequent communication between your browser and the server is secure and private.

4. Sending the HTTP/HTTPS Request

Once a secure (for HTTPS) or unsecure (for HTTP) connection is established, your browser sends an HTTP (Hypertext Transfer Protocol) or HTTPS request to the server. This request includes:

• HTTP Method: Such as GET (to request a resource) or POST (to submit data).
• Path: The specific resource being requested (e.g., /index.html or /products/shoes).
• Headers: Additional information like the browser type, preferred language, and cookies.

5. Server Processing and Response

The web server receives your request, processes it, and generates a response.

• Resource Retrieval: The server locates the requested file or executes a script (like PHP, Python, Node.js) to dynamically generate content.
• Database Interaction: If necessary, the server might interact with databases to fetch or store data.
• Response Generation: The server then constructs an HTTP/HTTPS response message, which includes:
  • HTTP Status Code: A numerical code indicating the outcome of the request (e.g., 200 OK for success, 404 Not Found, 500 Internal Server Error).
  • Headers: Information about the server, content type, and caching instructions.
  • Body: The actual content of the web page (HTML, CSS, JavaScript, images, videos, etc.).

6. Browser Rendering

Finally, your browser receives the server's response and begins the rendering process to display the web page.

• Parsing HTML: The browser parses the HTML to construct the Document Object Model (DOM), which is a tree-like representation of the page's structure.
• Fetching Additional Resources: As the browser parses HTML, it discovers references to other resources like CSS files (for styling), JavaScript files (for interactivity), images, and fonts. It then initiates new requests (often parallel to speed up loading) for these resources, repeating the DNS lookup and connection process if necessary.
• Applying Styles: CSS files are parsed and applied to style the HTML elements.
• Executing Scripts: JavaScript files are executed, which can modify the DOM, add interactive elements, or fetch more data.
• Layout and Painting: The browser calculates the layout of the elements and then "paints" them onto the screen, making the web page visible to you.

Key Components Involved

Understanding these components provides a clearer picture of the intricate process.

This complex but highly efficient ballet of technologies ensures that when you type in a URL, the desired content appears on your screen within moments.