Choose: Host your mobile version of your website under a seperate domain (.mobi), or to use the same domain (.com,.org.,,) of your original website?

The best practice for creating a mobile version of your website is to use the same domain for all devices and write a device detector in your controller (if using MVC) or in the header of every page you have.

this will display a device based webpages under the same host-domian.

I will choose same domain for both desktop and mobile versions. That was very old practice to create two different sites for desktop and mobiles.

We have HTML5 and CSS3 nowadays which open a new world in webdevelopment. We can make sites for both types in one go. No more need to create two different websites for them.

So why does Microsoft have two seperated domains ? (sub domain for mobile version)http://www.microsoft.comhttp://m.microsoft.comif I open the main website of microsoft from my iphone it will not redirect  to the mobile version?

It is true that HTML5 and CSS3 enable us now to develop the same site targeting different platforms(mobile and desktop) But there is another bitter truth and that is, Not all browsers support the HTML5 and CSS3 in same way and in full capability. Even Safari behaves differently on mobile and does not provide same feature set that is provided on desktop version. Another problem is the screen size. In desktop version we have large area to show and we can put as many info on screen as desired but on mobile we are again stuck because of the screen size and we only have to show the information which is relatively more important.

So IMHO, we should'nt have the same site targeting different platforms. Yes we can have same business logic but views should be different.

As Mr. AlGanneh said, there's normally no need for  another domain, through responsive design combining media qeuries and device detection you can have your website working seemlessly on mobile,tablets,and PCs.