What happens when we type a URL in the browser (In Depth)?

If you are reading this then i would recommend to read this  article first for a clear visibility that what we will understand in depth 🙂


Have you ever wondered, precisely – 

  • What happens in the background when we type a URL in the browser?
  • How does a web page life-cycle sequence work?
  • How browser knows, where to send request for the requested page?
  • What happens on the Web Server when a request for a web page comes in?
  • How does the Web server handle the incoming request?
  • How is the HTML that is emitted to the client generated?
  • How browser renders the page on the screen?
  • Etc…

If you are also looking answers of above mentioned questions then this article is definitely for you. In this article, we will take a deeper look at the sequence of events that take place when we visit a ASP.NET page URL –

  1. URL is typed in address bar of the browser
  2. The browser parses the URL to find the protocol, host, port, and path. Ex- http://www.jogendras.com/about will be parsed as-
  3. Browser checks cache, if requested object is in cache and is fresh then return response from cache.
  4. The browser looks up the IP address for the entered domain name.  When we want to connect to http://www.google.com, we actually want to reach out to a server where web application is hosted. One such server is having an IP address of XX.XXX.XXX.XX. Now, if we type http://XX.XXX.XXX.XX in the browser, this will take us to http://www.google.com itself. Which means, http://www.google.com and http://XX.XXX.XXX.XX are nothing but same stuff. But, it is not so. 
    As google has multiple servers in multiple locations to cater to the huge volume of requests. Thus we should let application decide which server is best suited to our needs. Using http://www.google.com does the job for us. When we type http://www.google.com DNS services comes into play and resolves the URL to a proper IP address. The DNS lookup proceeds as follows-

    • Browser cache – The browser maintain cache of DNS records for some time. So, this is the first place to resolve DNS queries
    • OS cache – If browser doesn’t contain the record in its cache, it makes a system call to underlying Operating System to fetch the record as OS also maintains a cache of recent DNS queries.
    • Router cache – If above steps fail to get a DNS record, the search continues to your router which has its own cache
    • ISP DNS cache – If above also fails then search moves on to ISP’s DNS server, first, it tries in its cache if not found then ISP’s DNS recursive search comes into the picture.
    • Recursive search – If everything fails then recursive search begins from the root level namesever.  For the DNS enthusiasts – here is a great guide worth reading
  5. Browser asks OS for entered domain’s IP address
  6. OS makes a DNS lookup and replies the IP address to the browser


Coming Soon….. 🙂


Programming is Easy…

What happens when we type a URL in the browser?

If you are curious to know what happens in the background when we type a URL in the browser then this article is definitely for you. Below are the sequence of events that occurs when we enter a URL in the browser’s address bar-

  1. You type the URL into address bar in your preferred browser
  2. The browser parses the URL to find the protocol, host, port, and path.
  3. Browser checks cache, if requested object is in cache and is fresh, skip to #15
  4. Browser asks OS for entered domain’s IP address
  5. OS makes a DNS lookup and replies the IP address to the browser
  6. Browser opens a TCP connection to the server
  7. When a connection is open, browser sends the HTTP request to the host
  8. The host forwards the request to the server software (most often Apache, IIS) configured to listen on the specified port
  9. Server handles the incoming request i.e server inspects the request (most often only the path), and launches the server plugin needed to handle the request (corresponding to the server language you use, PHP, Java, .NET, Python?)
  10. The server plugin gets access to the full request, and starts to prepare a HTTP response.
  11. Server sends the HTTP response back to the browser
  12. Browser receives HTTP response and may close the TCP connection, or reuse it for another request
  13. Browser checks if the response is a redirect or a conditional response (3xx result status codes), authorization request (401), error (4xx and 5xx), etc.; these are handled differently from normal responses (2xx)
  14. If cache-able, response is stored in cache
  15. Browser decodes response (e.g. if it’s gzipped)
  16. Browser determines what to do with response (e.g. is it a HTML page, is it an image, is it a sound clip?) Browsers often use the MIME-type to determine what default action to do when a resource is fetched.
  17. If response is HTML then browser parses the HTML. A DOM tree is built out of the response HTML. New requests are made to the server for each new resource that is found in the HTML source (typically images, style sheets, and JavaScript files). Go back to step#7 and repeat for each resource. StyleSheets are parsed, and the rendering information in each gets attached to the matching node in the DOM tree. Javascript is parsed and executed, and DOM nodes are moved and style information is updated accordingly. The browser renders the page on the screen according to the DOM tree and the style information for each node
  18. Browser renders response, or offers a download dialog for unrecognized types


Hopefully this gives you a better idea of how a URL get proceeds 🙂


Programming is Easy…