If you have several domains with different extensions and you need all of them to open the same website, you can have the website under one of them and forward the rest. There are various ways to forward one domain name to another, such as the so-called domain parking. When your web hosting package 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 difference between the two is that while a domain is hosted, you can still have content for it, create subdomains, email addresses, etc., while with a parked domain you can't do any of those things. As an illustration, if you're building localized Internet sites under various country-code domains, you will be able to work on them, but in the meantime, visitors will be redirected to the primary website.