What Happens When You Type a URL in a Browser?

TL;DR

• When you press Enter, your browser looks up the site's address (DNS), opens a connection (TCP), sets up encryption (TLS), asks for the page (HTTP), then builds and draws it (rendering) [1].
• Most of this takes a fraction of a second, because browsers reuse and cache a lot.
• Each step needs back-and-forth trips over the network, which is why distance and slow connections feel slow.

The five steps #

A URL such as https://example.com/page contains the protocol (https), the host name (example.com) and the path (/page). Computers do not find sites by name, so the browser has to turn that name into a route to a real machine and then ask it for the page. The diagram shows the order.

Step 1: Finding the server with DNS #

The browser needs a numeric IP address. It first checks its own saved answers. If it does not know, it asks the Domain Name System (DNS), which maps friendly names like mozilla.org to addresses [3]. Think of looking up a name in a phone book to get a number. Every separate host name used by a page, such as fonts or scripts from other sites, needs its own lookup [1].

Step 2: Opening a connection (TCP) #

With the address, the browser opens a connection to the server using a three-part handshake, a short exchange to agree how the two sides will talk before any real request is sent [1]. HTTP normally runs on top of this kind of reliable connection [2].

Step 3: Setting up encryption (TLS) #

For an https address the two sides also agree how to encrypt the conversation, and the browser checks the server's identity. This takes extra round trips [1]. Newer setups can shorten the process, but the idea is the same: a private channel before any page data moves.

Step 4: Asking for the page (HTTP) #

Now the browser sends a request. In plain terms it says "please GET this page", plus a few details in headers. The server answers with a status code, such as 200 for success, headers and usually the HTML of the page [2].

Step 5: Turning code into a page #

The HTML is only text. The browser parses it into a tree of elements called the DOM and turns the CSS into a matching style tree. It combines the two, works out the size and position of every box (layout), then paints pixels on the screen [1]. As it reads the HTML it finds images, styles and scripts and requests each of those too [2]. Large scripts can delay the moment the page responds to clicks [1].

FAQ #

What is the difference between a URL and a domain name? #

The domain name is just the site's name, like example.com. The URL is the full address, including the protocol and path, such as https://example.com/page.

Why do some pages load faster the second time? #

Browsers save some answers and files, including DNS results and images, so they can skip steps. They also often reuse an existing connection [2].

Where does a CDN fit in? #

A CDN can answer step 4 from a server close to you instead of the site's main server, which cuts the distance each trip has to travel.

What if the DNS lookup fails? #

The browser cannot find where to connect, so you usually see an error saying the site cannot be reached, even if the server itself is fine.

Sources #