If you have a lot of domains with different extensions and you need all of them to open the very same website, you could have the website under one of them and forward the rest. There are several ways to redirect one domain to another, like the so-called domain parking. If your web hosting package allows it, though, it'll be better if you host all the domains and create a URL redirect, not a domain redirect. The difference between the two is that while a domain address is hosted, you can still have content for it, create subdomains, e-mail addresses, etc., while with a parked domain you can't do any of those things. For instance, if you're building localized websites under different country-code Internet domain names, you'll be able to work on them, but at the same time, people will be forwarded to the primary site.