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

As the topic is self explanatory - when we hit any URL then what happens? - let’s start!
Before discussing what happens after hitting the URL, we must go through what a URL actually is, and what different parts of the URL mean - right? Without wasting any time, let’s understand more about URLs.

URL – Uniform Resource Locator

If you look into its full form, then it is self explanatory: it has the location of the resources which we want to access. It is an address of the place where we want to go to interact with or find information.
Let's look into your daily life. If you want to visit your friend’s house for some work or to get information, you need their address. The same thing goes here in this big web world: we have to give an address of the website which we want to access. The web site is like the house and the URL is the address.

Anatomy of a URL

Now, we know what a URL is but we still don’t know about the parts of a URL. Let’s go!
Let’s take an example:
https://www.example.com/page1
Here, the first part is ‘https’. This basically tells the browser which protocol it should use. It can be http, https, ftp, etc. A protocol is a set of rules that browser use for communication over the network. 'https' is basically a secure version, i.e. information is exchanged in a secure way.
The second part www.example.com is a domain name. You can relate it to your friend’s house. It is an address of website. We use it to reach to the server (trained computer) that is responsible for serving the information for that website. Wait! You might think, a seconds before I mentioned URL is the address whereas I also mentioned domain name is also address. You may have been confused. Don’t be confused!

Difference between URL and Domain Name

The major difference between both is that the URL is a complete address. URL tells about the method through which information should exchange, the path after reaching that website. Whereas the domain name is part of a URL.
Let’s take our previous example to better understand. You can say that your friend’s house address is a domain name, whereas the URL not only tells the friend’s house address (domain name) but also how you are going to communicate like talking in a separate room (secure) or in front of everyone (info can get leak). It also tells the path, i.e. at which part of the house you will go after entering into the house. Hence, the domain name is part of the URL. A domain name with more information is a URL.
I hope now you are clear with the URL. Let’s get into the next part.

Domain Name

In the previous part, I explained about domain names, but not in depth. I want you to go into it more. As I told you, the Domain name is the address of the website. It gives a unique identity to your website in such a huge web world. No two domain names can be the same BUT - Yes! There is ‘but’. This is not the only definition of a domain name. There is another story behind it. Let’s get into that story.
As we know, when we hit any URL or you can say domain name, then that website gets opened with its content. A server (a trained computer) serves it. We also know that every computer has an IP address which is used for communication over the internet. It is an address as its self explaining ‘IP address’. When we hit any URL, then we are actually hitting the IP address of the computer which is responsible for serving the website content (hosting).
But, now, you might think what the hell...is everything an address? Why does this domain name exist if the IP address is there? Why can’t we use IP address to get content of the website?
Yes! You can use IP addresses to get content of the website but really!.. Would you be able to remember each website’s associated IP address? Obviously not! It’s hard to remember the IP address of every website. That’s why domain names came into the market.
You can relate it to your contact list. You can’t remember every person’s number, but you can remember their name. Same concept applies here as well. You can’t remember those scary IP addresses, but you can easily remember domain names.
This huge amount of data is maintained in a database where the domain name with its IP address is stored. A system that stores domain names with its corresponding IP address is known as DNS (Domain name system) (I believe you must have heard about it).
I think I have discussed enough basics. Now, get a deep dive into the process of when we hit any URL.

DNS lookup to find IP address

After hitting the URL, the first thing that needs to happen is to resolve IP address associated with the domain name. DNS helps in resolving this. DNS is like a phone book and helps us to provide the IP address that is associated with the domain name just like our phone book gives a mobile number which is associated with the person’s name.

This is the overview, but there are four layers through which this domain name query goes through. Let’s understand the steps:
1. After hitting the URL, the browser cache is checked. As browser maintains its DNS records for some amount of time for the websites you have visited earlier. Hence, firstly, DNS query runs here to find the IP address associated with the domain name.
2. The second place where DNS query runs in OS cache followed by router cache.
3. If in the above steps, a DNS query does not get resolved, then it takes the help of resolver server. Resolver server is nothing but your ISP (Internet service provider). The query is sent to ISP where DNS query runs in ISP cache.
4. If in 3rd steps as well, no results found, then request sends to top or root server of the DNS hierarchy. There it never happens that it says no results found, but actually it tells, from where this information you can get. If you are searching IP address of the top level domain (.com,.net,.Gov,. org). It tells the resolver server to search TLD server (Top level domain).
5. Now, resolver asks TLD server to give IP address of our domain name. TLD stores address information of domain name. It tells the resolver to ask it to Authoritative Name server.
6. The authoritative name server is responsible for knowing everything about the domain name. Finally, resolver (ISP) gets the IP address associated with the domain name and sends it back to the browser.
After getting an IP address, resolver stores it in its cache so that next time, if the same query comes then it does not have to go to all these steps again. It can now provide IP address from their cache.
This is all about the steps that is followed to resolve IP address that is associated with the domain name. Have a look below to better understand:

TCP connection initiates with the server by Browser

Once the IP address of the computer (where your website information is there) is found, it initiates connection with it. To communicate over the network, internet protocol is followed. TCP/IP is most common protocol. A connection is built between two using a process called ‘TCP 3-way handshake’. Let’s understand the process in brief:
1. A client computer sends a SYN message means, whether second computer is open for new connection or not.
2. Then another computer, if open for new connection, it sends acknowledge message with SYN message as well.
3. After this, first computer receives its message and acknowledge by sending an ACK message.
To better  understand, look below diagram.

Communication Starts (Request Response Process)

Finally, the connection is built between client and server. Now, they both can communicate with each other and share information. After successful connection, browser (client) sends a request to a server that I want this content. The server knows everything of what response it should send for every request. Hence, the server responds back. This response contains every information that you requested like web page, status-code, cache-control, etc. Now, the browser renders the content that has been requested.
That’s it! All the above process happens when we hit any URL. Although this lengthy process takes less than seconds to complete. This is the answer to your question ‘what happens when we hit any URL in a browser?’

It is always good to have a basic understanding of what is going on behind our browsers and how information is transferred to our computers via the internet.

Let’s imagine that you want to access maps.google.com to check the exact time it would take for you to get to your dinner reservation from work.

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

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

DNS(Domain Name System) is a database that maintains the name of the website (URL) and the particular IP address it links to. Every single URL on the internet has a unique IP address assigned to it. The IP address belongs to the computer which hosts the server of the website we are requesting to access. For an example, has an IP address of 209.85.227.104. So if you’d like you can reach by typing on your browser. DNS is a list of URLs and their IP addresses just like how a phone book is a list of names and their corresponding phone numbers.

The main purpose of DNS is human-friendly navigation. You can easily access a website by typing the correct IP address for it on your browser but imagine having to remember different sets of numbers for all the websites we regularly access? Therefore, it is easier to remember the name of the website using an URL and let DNS do the work for us with mapping it to the correct IP.

In order to find the DNS record, the browser checks four caches.

● First, it checks the browser cache. The browser maintains a repository of DNS records for a fixed duration for websites you have previously visited. So, it is the first place to run a DNS query.

● Second, the browser checks the OS cache. If it is not found in the browser cache, the browser would make a system call (i.e. gethostname on Windows) to your underlying computer OS to fetch the record since the OS also maintains a cache of DNS records.

● Third, it checks the router cache. If it’s not found on your computer, the browser would communicate with the router that maintains its’ own cache of DNS records.

● Fourth, it checks the ISP cache. If all steps fail, the browser would 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.

You may wonder why there are so many caches maintained at so many levels. Although our information being cached somewhere doesn’t make us feel very comfortable when it comes to privacy, caches are important for regulating network traffic and improving data transfer times.

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.

As mentioned earlier, in order for my computer to connect with the server that hosts maps.google.com, I need the IP address of maps.google.com. The purpose of a DNS query is to search multiple DNS servers on the internet until it finds the correct IP address for the website. This type of search is called a recursive search since the search will continue repeatedly from DNS server to DNS server until it either finds the IP address we need or returns an error response saying it was unable to find it.

In this situation, we would call the ISP’s DNS server a DNS recursor whose responsibility is to find the proper IP address of the intended domain name by asking other DNS servers on the internet for an answer. The other DNS servers are called name servers since they perform a DNS search based on the domain architecture of the website domain name.

Without further confusing you, I’d like to use the following diagram to explain the domain architecture.

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 .com domain name server. .com name server will redirect it to google.com name server. 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.

These requests are sent using small data packets which contain information such as the content of the request and the IP address it is destined for (IP address of the DNS recursor). These packets travel through multiple networking equipment between the client and the server before it reaches the correct DNS server. This equipment use routing tables to figure out which way is the fastest possible way for the packet to reach its’ destination. If these packets get lost you’ll get a request failed error. Otherwise, they will reach the correct DNS server, grab the correct IP address, and come back to your browser.

4. 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 IP address to transfer information. Browsers use internet protocols to build such connections. There are a number of different internet protocols which can be used but TCP is the most common protocol used for any type of HTTP request.

In order 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.

1. Client machine sends a SYN packet to the server over the internet asking if it is open for new connections.

2. 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.

3. 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 web server.

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.

Sample GET request (Headers are highlighted):

(If you’re curious about what’s going on behind the scenes you can use tools such as Firebug to take a look at HTTP requests. It is always fun to see the information passed between clients and servers without us knowing).

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

The server contains a web server (i.e Apache, IIS) which 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 sends 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. At 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 happens within milliseconds before we could even notice. I sincerely hope this article helps you answer the question “What happens when you type an URL in the browser and press enter?”.

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