What happens when we type a
URL in our browser and press
1. Parse the URL
To visit any website on the Internet we need to know the
address of the
server that is hosting that website. That address is a number called the
IP stands for
Internet Protocol. A
protocol is a set of rules that define a method of exchanging information over a
computer network. That is what the
internet really is, a network made up of billions of connected computers, each one with their own
google.com‘s IP is
188.8.131.52, so by typing
http://184.108.40.206 in your browser you get to google.com.
The first thing the browser does is check if what we entered is an
IP. If it is, then it will try to connect to it. In reality though, it’s impossible for us to remember all the
IPs of the many websites we visit, so we remember their
URLs instead. But URLs are not addresses and our browser needs the exact address. So how does it figure that out? Naturally, it looks it up!
2. Lookup Address
There are several different locations where the
IP address can be found. First the browser checks these 2 locations on its computer.
The browser searches its cache.
Cache (pronounced cash) is a hardware or software component that stores data so future requests for that data can be served faster. The
Chrome browser, for example, caches the IPs of the URLs you visit for 30 seconds, after that the cache expires. If you use Chrome you can type in this URL to see your cached addresses
Second, the browser looks in the operating system’s
hosts file. The hosts file is a text document that has a list of IPs with their associated domains.
If the IP address cannot be found locally, the browser must do an external search. This is where
DNS comes to the rescue!
It stands for
Domain Name Service, which is a
database hosted on multiple servers around the world that contains
records of every
domain on the internet and its
DNS makes browsing the internet human-friendly, as we no longer have to remember any numbers, instead remembering the
URL name, and
DNS gives us the
IP. DNS lookup proceeds as follows:
IP is not found on the browser’s computer, the browser tries to see if there’s a
router on the
network with a DNS cache. If it doesn’t find it there it connects to the ISP.
ISP DNS Cache
ISP stands for
Internet Service Provider, which is the company you pay monthly to have Internet access.
ISPs have DNS cache servers in their data centers. Those servers are the next thing to get checked. If the record is still not found, the ISP’s DNS server initiates a
recursive DNS query.
Recursive DNS Query
Simply put, a recursive DNS query is multiple DNS servers calling each other until they find the correct record, which is then returned to the browser.
Let’s say we’re trying to find the IP of
mail.google.com. The ISP’s DNS server contacts the top-level domain
.com DNS server, which redirects to the
google.com DNS server, which returns the IP of
mail.google.com all the way back to the browser.
Note that even if we have to go through all these steps to get the IP, it all happens extremely quickly and we don’t have to do any kind of waiting usually. Also, the DNS servers will cache any IP they did not have for a certain amount of time, so it can be accessed quickly the next time it’s requested.
3. Connect to Server
Initiate TCP/IP Connection
Once the browser (which is the
client in a
server/client relationship) receives the IP, it will attempt to establish a connection to the website’s hosting server using
TCP (Transmission Control Protocol).
The connection is established using a
TCP/IP 3-way handshake.
Before we get to the server we usually go through some network components, most importantly being the load balancer and the firewall described below.
1. Client asks the server if it is open for connections by sending it a
SYN (synchronize) packet.
2. If the server can accept the connection it responds with an
ACK (Acknowledgement) of the SYN by sending a
3. Client receives SYN/ACK packet from server and acknowledges by sending another
Now we have an
established TCP/IP connection and we can
transfer data back and forth!
IP address we get for the website we’re trying to visit usually belongs to a server called the
load balancer. The load balancer does what you imagine, it splits up the load or web traffic onto multiple servers. Large website that receive a lot of traffic need to do this because one server is not enough for the potential millions of users trying to connect.
Sometimes the IP we get belongs to a
firewall which is a very important
security component in the
network stack. A firewall can be implemented as software or hardware. Firewalls exist at different locations in the network, either before or after load balancers. The firewall is a barrier between a trusted and an untrusted network. It has rules that define who is allowed to access the network and who will be blocked.
Websites store their information in an
SQL database. Larger websites split their database onto multiple servers.
SQL (Structured Query Language) is used to manipulate the database. SQL commands such as
Drop are used to accomplish almost everything one needs to do with a database.
4. HTTP(S) Request
HTTP is the
Hypertext Transfer Protocol. It is the underlying protocol used by the
World Wide Web to define how messages are formatted and transmitted between a Web server and a browser.
If you look closely at the displayed URL in your browser, you see it often starting with
http. HTTPS is the secure version of the HTTP protocol. That means that the connection between us and the server is encrypted and can’t be deciphered by anyone listening in to our communication. This is very important when you are dealing with sensitive information. You don’t want anyone finding out your banking, credit card or social security number, for example.
HTTPS uses the
SSL (Secure Sockets Layer) protocol to establish an encrypted connection between a web server and a browser. An SSL certificate is necessary to create SSL connection. You can see the SSL certificates being used by Chrome by clicking on
Manage certificates under the
Privacy and security preferences.
Once the TCP/IP connection is established, the browser uses HTTP to send a GET request to the web server. The server has a software running on it called a
web server like
Nginx that processes the GET request and sends the needed information back.
Sometimes we need to send information to the server, like login info, or we might be submitting a form. In this case the browser will send an
HTTP POST request to the server instead of a GET request. To see the client/server communication happening in the background when you browse the internet checkout Firefox’s
Firebug plugin. A lot more communication is happening that what you’d expect!
5. Server Response
Server Response Codes
A server has multiple kinds of responses to the GET/POST requests it receives. The response is a 3-digit number, the first one of which tells us what family of messages the response belongs to. The one we’re usually familiar with is the famous
404 Page Not Found.
It breaks down like this:
1xx: Informational response
4xx: Client error
5xx: Server error
6. Display Webpage
Once the basic HTML code is received it is processed and displayed by the browser. After that the browser sends GET requests for other items that the HTML code refers to, like
CSS style guides,
videos. That’s why a lot of times it seems to us that the page loads in parts, with components with the biggest size loading last.