Since you don't say which version of DNN you are using, I offer instructions for Ver 6.02.02.

You need multiple domain names all pointed at the root folder of the dnn install, and then log in as host, go to site management and click the Manage button. Select new site, add the domain name as the site alias, best to leave the Portals/[PortalId] as the home directory. Give the site a title, description and keywords are optional, and I usually accept defaults on the last two.

Bill

Yes it is ok right all are thing now i m using DNN 6.2.2 .I have created portals or multiple sites. just one thing i want ask one question if i added new features like as menu item then if i want  these particular features only displayed on selected particular portals or domains .so how we can be implemented these functionality nd it is possible with DNN or not?