If you have several domains with different extensions and you want all of them to open the very same site, you can have the website under one of them and redirect the others. There are a number of ways to redirect one domain name to another, like the so-called domain parking. When your website hosting plan allows 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 can still have content for it, create subdomains, email addresses, etc., while with a parked domain you cannot do any of these things. As an illustration, if you are building localized Internet sites under different country-code domain names, you shall be able to work on them, but at the same time, people will be redirected to the main Internet site.