What Happens Behind the Scenes When You Enter a Website URL?

Introduction

Have you ever wondered what actually happens when you type a website address such as https://just-programmer.ir into your browser and press Enter?

From the outside, it looks simple: you enter an address and a webpage appears. Behind the scenes, however, the browser communicates with DNS services, network systems, web servers, and its rendering engine to retrieve website resources and turn them into a visible webpage.

From resolving a domain name to establishing a secure connection, sending an HTTP request, receiving HTML files, and rendering the final page, several different processes work together.

In this article from Just Programmer, we will explore what happens when you visit a website and how a browser turns a URL into a webpage.

1. Entering a Website URL

The process begins when a user enters a website address into the browser's address bar.

For example:

https://just-programmer.ir

A URL contains different parts:

• HTTPS: The protocol used for secure communication.

• just-programmer.ir: The website's domain name.

• Path: The path to a specific resource, if present.

• Query Parameters: Additional parameters in the URL, if present.

The browser parses the URL to determine which resource it needs to access and what type of connection is required.

For example:

https://just-programmer.ir/articles

In this case, the browser considers both the domain and the /articles path when creating the request.

2. What Is DNS and How Does It Resolve a Domain?

Computers use IP addresses to communicate with destinations across a network. However, remembering IP addresses is difficult for people, so domain names are used instead.

For example, instead of entering an IP address, we use:

just-programmer.ir

This is where DNS becomes important.

DNS stands for Domain Name System. It provides a system for resolving domain names into information needed to locate a network destination, including IP addresses.

A Simplified DNS Lookup Process

When a browser needs an IP address, the system may follow steps such as:

1. Checking the browser cache.

2. Checking operating system or local resolver caches.

3. Sending a request to a DNS Resolver if necessary.

4. The resolver checking cached information or querying DNS servers.

5. Receiving a response for the domain.

6. Using the response to connect to the destination.

In practice, not every request needs to go through every step because DNS information may already be cached.

Example

Domain:
just-programmer.ir

DNS:
Domain → IP Address

A domain can point to one or multiple IP addresses, and DNS responses do not always return a single fixed IP address.

3. What Is an IP Address and Why Is It Needed?

An IP address is a network identifier used to identify a destination on an IP network.

The two main versions are:

• IPv4: For example, 192.168.1.1.

• IPv6: For example, 2001:db8::1.

Once the browser has the information needed to locate the destination, it can attempt to establish a connection with the server.

DNS itself does not transfer the webpage content. Its role is to help resolve the destination information.

4. Establishing a Connection with the Server

After the destination is determined, the browser needs to establish a suitable network connection.

The connection process depends on the transport protocol being used.

TCP for HTTP/1.1 and HTTP/2

For TCP-based connections, a TCP connection is generally established first. This process is known as the TCP Three-Way Handshake.

The simplified steps are:

1. SYN: The client sends a request to start a connection.

2. SYN-ACK: The server responds to the request.

3. ACK: The client sends the final acknowledgment, and the connection is established.

If the website uses HTTPS, the TLS security setup is performed after the TCP connection is established.

Does Every Website Use TCP?

No. HTTP/3 uses QUIC over UDP, and its connection process differs from TCP. Therefore, the exact connection steps depend on the HTTP version and transport protocol.

5. How Do HTTPS and TLS Protect Communication?

When a website uses HTTPS, the browser establishes a secure TLS connection.

TLS stands for Transport Layer Security. It is a protocol designed to provide security for network communication.

HTTPS is HTTP transported over a secure TLS connection.

What Does TLS Provide?

TLS is designed to provide important security properties, including:

• Encryption: Protecting the confidentiality of data in transit.

• Server Authentication: Helping verify the identity of a server through certificates and trust chains.

• Data Integrity: Helping detect unauthorized changes to transmitted data.

What Is a TLS Handshake?

During a TLS handshake, the client and server exchange information needed to establish a secure connection.

In TLS 1.3, the handshake includes messages used to negotiate security parameters, authenticate the server, and establish cryptographic keys.

After the secure connection is established, HTTP data can be transmitted through the TLS-protected channel.

Important: HTTPS does not automatically mean that every part of a website or its content is trustworthy. HTTPS protects communication, while application security and content reliability are separate concerns.

6. The Browser Sends an HTTP Request

Once the connection is ready, the browser sends an HTTP request to retrieve the requested resource.

For example:

GET / HTTP/1.1
Host: just-programmer.ir
Accept: text/html

This is a simplified example of an HTTP request. A real request may contain additional headers, and its structure depends on the HTTP version and browser conditions.

Main Parts of an HTTP Request

• Method: The requested operation, such as GET or POST.

• Path: The resource path on the server.

• Headers: Additional information about the request.

• Body: Request data, when needed.

GET is commonly used to retrieve resources, while POST can be used to send data to a server.

7. How Does the Server Process the Request?

After the request reaches the server, the web server or relevant infrastructure receives and processes it.

Depending on the website architecture, the following steps may occur:

1. The request is received by a web server or reverse proxy.

2. The route and request method are evaluated.

3. The request is forwarded to a backend application if necessary.

4. The application logic is executed.

5. Data is retrieved from a database or other services if needed.

6. An HTTP response is generated.

7. The response is sent back to the client.

For example, a website built with Flask may receive requests through a web server or WSGI server before the request reaches the Flask application.

A simple Flask example:

from flask import Flask

app = Flask(__name__)

@app.route("/")
def home():
    return "<h1>Hello from Flask!</h1>"

When a request reaches the / route, the home() function is executed and its response is returned to the client.

Real-world applications may include additional layers such as authentication, databases, middleware, and reverse proxies.

8. What Is an HTTP Response?

After processing the request, the server sends an HTTP response.

A simplified HTTP response might look like this:

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

<h1>Hello World!</h1>

Main Parts of a Response

• Status Code: The result of processing the request.

• Response Headers: Information about the response.

• Response Body: The content returned to the client.

Common HTTP Status Codes

HTTP status codes are divided into different categories, including 1xx, 2xx, 3xx, 4xx, and 5xx.

9. The Browser Receives HTML, CSS, and JavaScript

The initial website response often contains HTML. The browser processes the HTML and may send additional requests for other required resources.

For example:

<!DOCTYPE html>
<html lang="en">
<head>
    <link rel="stylesheet" href="/static/style.css">
</head>
<body>
    <h1>Hello World</h1>

    <script src="/static/app.js"></script>
</body>
</html>

In this example:

• CSS is used to style the page.

• JavaScript is used to add interactive behavior.

• HTML defines the main page structure.

The browser may send additional HTTP requests to retrieve these resources.

Resources may include:

• CSS files.

• JavaScript files.

• Images.

• Fonts.

• Videos.

• Application data.

The loading and prioritization of resources depend on the HTML elements, their attributes, and browser behavior.

10. How Does the Browser Turn HTML and CSS into a Webpage?

After receiving the resources, the browser needs to process them so that the page can be displayed.

This process involves several important stages.

HTML Parsing

The browser parses HTML and builds a DOM representation.

CSS Parsing

CSS files are processed, and style information is organized into internal structures.

Style Calculation

The browser determines which styles apply to each element.

Layout

The browser calculates the size and position of elements.

Paint

The visual content of elements is prepared for drawing.

Compositing

When needed, different layers are combined for final presentation.

These stages are simplified, and the exact implementation can vary between browsers.

11. When Does JavaScript Execute?

JavaScript can execute at different stages of page loading and interaction.

For example:

<script src="app.js"></script>

The execution behavior depends on attributes such as defer and async, the script's location, and the document's loading state.

The defer Attribute

<script src="app.js" defer></script>

A script with defer can be downloaded without blocking HTML parsing and generally executes after HTML parsing is complete, following the rules for deferred scripts.

The async Attribute

<script src="app.js" async></script>

A script with async can execute after it has been downloaded, independently of whether HTML parsing has finished. The execution order of multiple async scripts is not guaranteed.

The choice between defer and async depends on the script's dependencies and the needs of the application.

12. What Is Browser Cache and Why Does It Matter?

Browsers can store certain resources in a cache to reduce unnecessary requests and improve loading performance.

Cached resources may include:

• CSS files.

• JavaScript files.

• Images.

• Fonts.

• Cacheable HTTP responses.

If a resource is available in the cache and the caching rules allow it to be reused, the browser may use the stored version instead of downloading it again.

Benefits of Caching

• Reduced resource loading time.

• Lower bandwidth usage.

• Fewer network requests.

• Improved user experience.

Caching does not always mean that a stored resource can be used without checking. HTTP headers and validation rules determine how the browser handles cached resources.

13. Does a CDN Help Load Websites?

CDN stands for Content Delivery Network. It is a network of servers distributed across different geographical locations.

A website may use a CDN to deliver static resources such as images, CSS, and JavaScript files.

When a user requests a resource, a CDN may serve it from an appropriate location if the resource is available in its cache.

Potential Benefits of a CDN

• Reducing network distance between users and resources.

• Improving delivery time for certain files.

• Reducing load on the origin server.

• Increasing the capacity for delivering cached content.

A CDN does not necessarily serve every request from its cache. Its behavior depends on configuration, user location, resource type, and network conditions.

14. Why Do Some Websites Load Faster Than Others?

Website loading speed depends on many different factors.

Important factors include:

• The user's internet connection.

• DNS resolution time.

• Network distance to the server.

• Connection establishment time.

• Backend processing time.

• Database performance.

• The size of HTML, CSS, and JavaScript files.

• The number of network requests.

• Cache and CDN usage.

• Browser and device performance.

For example, if a website's backend takes a long time to retrieve data from a database, the HTTP response may take longer to become available.

Even if the server responds quickly, large files or expensive JavaScript operations can delay page rendering and interaction.

For this reason, website performance optimization is not limited to making the server faster.

15. Conclusion

When we enter a website address into a browser, a multi-step process begins behind the scenes.

First, the browser parses the URL and, when necessary, uses DNS to resolve information about the destination. Depending on the protocol, a network connection and secure TLS communication are established.

The browser then sends an HTTP request to the server. The server processes the request and returns a response.

The browser receives and processes resources such as HTML, CSS, and JavaScript, and eventually renders the webpage for the user.

From entering a simple URL to displaying a complete website, networking, server infrastructure, browser technology, and programming all work together.

Understanding this process helps developers troubleshoot website issues, improve application performance, and gain a deeper understanding of how the web works.

Frequently Asked Questions (FAQ)

What is DNS?

DNS, or Domain Name System, is a system used to resolve domain names into information needed to locate a network destination, including IP addresses.

What is the difference between HTTP and HTTPS?

HTTPS is HTTP transported over a secure TLS connection. TLS is designed to provide confidentiality, data integrity, and server authentication.

Does every browser connection use TCP?

No. HTTP/1.1 and HTTP/2 are commonly used over TCP, while HTTP/3 uses QUIC over UDP.

Why do websites need DNS?

DNS allows users to use readable domain names instead of having to remember network addresses.

How does a CDN help a website?

A CDN can deliver content from distributed network locations, potentially reducing delivery time for certain resources and lowering load on the origin server. Its performance depends on the content and configuration.

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