If you have several domain names with different extensions and you want all of them to open precisely the same site, you can have the site under one of them and redirect the rest. There are different ways to redirect one domain name to another, like the so-called domain parking. When your website hosting plan permits it, though, it'll be better if you host all of the domains and set up a URL redirect, not a domain redirect. The main difference between the two is that while a domain name is hosted, you may still have content for it, create subdomains, email addresses, etc., while with a parked domain it's not possible to do any of those things. As an illustration, if you're building localized Internet sites under various country-code domain names, you shall be able to work on them, but meanwhile, visitors shall be redirected to the primary Internet site.