Internet

What happens when you type a URL in the browser and press enter?

1. You type maps.google.com into the address bar of your browser.

2. The browser checks the cache for a DNS (Domain Name System) record to find the corresponding IP address of maps.google.com.

To find the DNS record, the browser checks four caches.

a) First, it checks the browser cache.

b) If not found in previous step, the browser checks the OS cache.

c) If again not found in the previous step, it checks the router cache.

d) If all steps fail, the browser will move on to the ISP. Your ISP maintains its’ own DNS server, which includes a cache of DNS records, which the browser would check with the last hope of finding your requested URL.

3. If the requested URL is not in the cache, ISP’s DNS server initiates a DNS query to find the IP address of the server that hosts maps.google.com.

The following diagram explains the domain architecture:

ISPs DNS server is a recursor (i.e. can make queries to other similar servers to fetch the result). Many website URLs we encounter today contain a third-level domain, a second-level domain, and a top-level domain. Each of these levels contains their own name server, which is queried during the DNS lookup process.

For maps.google.com, first, the DNS recursor will contact the root name server. The root name server will redirect it to the .com domain name server. .com name server will redirect it to the google.com name server. The google.com name server will find the matching IP address for maps.google.com in its’ DNS records and return it to your DNS recursor, which will send it back to your browser. This process is further explained in detail with help of the following diagram and for any webpage hosted in AWS:

1. A user opens a web browser, enters www.example.com in the address bar, and presses Enter.

2. The request for www.example.com is routed to a DNS resolver, which is typically managed by the user's Internet service provider (ISP), such as a cable Internet provider, a DSL broadband provider, or a corporate network.

3. The DNS resolver for the ISP forwards the request for www.example.com to a DNS root name server.

4. The DNS resolver for the ISP forwards the request for www.example.com again, this time to one of the TLD name servers (top-level-domain) for .com domains. The name server for .com domains responds to the request with the names of the four Amazon Route 53 name servers that are associated with the example.com domain.

5. The DNS resolver for the ISP chooses an Amazon Route 53 name server (Authoritative name server) and forwards the request for www.example.com to that name server.

6. The Amazon Route 53 name server looks in the example.com hosted zone for the www.example.com record, gets the associated value, such as the IP address for a web server, 192.0.2.44, and returns the IP address to the DNS resolver.

7. The DNS resolver for the ISP finally has the IP address that the user needs. The resolver returns that value to the web browser. The DNS resolver also caches (stores) the IP address for example.com for an amount of time that you specify so that it can respond more quickly the next time someone browses to example.com. For more information, see time to live (TTL).

8. The web browser sends a request for www.example.com to the IP address that it got from the DNS resolver. This is where your content is, for example, a web server running on an Amazon EC2 instance or an Amazon S3 bucket that's configured as a website endpoint.

9. The web server or other resource at 192.0.2.44 returns the web page for www.example.com to the web browser, and the web browser displays the page.

4. The browser initiates a TCP connection with the server.

Once the browser receives the correct IP address, it will build a connection with the server that matches the IP address to transfer information. Browsers use internet protocols to build such connections. There are several different internet protocols that can be used, but TCP is the most common protocol used for many types of HTTP requests.

To transfer data packets between your computer(client) and the server, it is important to have a TCP connection established. This connection is established using a process called the TCP/IP three-way handshake. This is a three-step process where the client and the server exchange SYN(synchronize) and ACK(acknowledge) messages to establish a connection.

a) The client machine sends a SYN packet to the server over the internet, asking if it is open for new connections.

b) If the server has open ports that can accept and initiate new connections, it’ll respond with an ACKnowledgment of the SYN packet using a SYN/ACK packet.

c) The client will receive the SYN/ACK packet from the server and will acknowledge it by sending an ACK packet. Then a TCP connection is established for data transmission!

5. The browser sends an HTTP request to the webserver.

Once the TCP connection is established, it is time to start transferring data! The browser will send a GET request asking for maps.google.com web page. If you’re entering credentials or submitting a form, this could be a POST request. This request will also contain additional information such as browser identification (User-Agent header), types of requests that it will accept (Accept header), and connection headers asking it to keep the TCP connection alive for additional requests. It will also pass information taken from cookies the browser has in store for this domain.

6. The server handles the request and sends back a response.

The server contains a webserver (i.e., Apache, IIS) that receives the request from the browser and passes it to a request handler to read and generate a response. The request handler is a program (written in ASP.NET, PHP, Ruby, etc.) that reads the request, its’ headers, and cookies to check what is being requested and also update the information on the server if needed. Then it will assemble a response in a particular format (JSON, XML, HTML).

7. The server sends out an HTTP response.

The server response contains the web page you requested as well as the status code, compression type (Content-Encoding), how to cache the page (Cache-Control), any cookies to set, privacy information, etc.

Example HTTP server response:

If you look at the above response, the first line shows a status code. This is quite important as it tells us the status of the response. There are five types of statuses detailed using a numerical code.

● 1xx indicates an informational message only

● 2xx indicates success of some kind

● 3xx redirects the client to another URL

● 4xx indicates an error on the client’s part

● 5xx indicates an error on the server’s part

So, if you encountered an error, you can take a look at the HTTP response to check what type of status code you have received.

8. The browser displays the HTML content (for HTML responses, which is the most common).

The browser displays the HTML content in phases. First, it will render the bare bone HTML skeleton. Then it will check the HTML tags and send out GET requests for additional elements on the web page, such as images, CSS stylesheets, JavaScript files, etc. These static files are cached by the browser, so it doesn’t have to fetch them again the next time you visit the page. In the end, you’ll see maps.google.com appearing on your browser.

That’s it!

Although this seems like a very tedious prolonged process, we know that it takes less than seconds for a web page to render after we hit enter on our keyboard. All of these steps happen within milliseconds before we could even notice. I sincerely hope this article helps you answer the question, “What happens when you type a URL in the browser and press enter?”.