If you have several domains with different extensions and you want all of them to open the exact same website, you could have the website under one of them and forward the others. There are different approaches to redirect one domain name to another, among them the so-called domain parking. If your web hosting package permits it, though, it will be better if you host all the domains and set up a URL redirect, not a domain redirect. The primary difference between the two is that while a domain name is hosted, you could still have content for it, set up subdomains, e-mail addresses, etc., while with a parked domain you can't do any of those things. As an illustration, if you are building localized sites under several country-code domains, you shall be able to work on them, but meanwhile, people will be forwarded to the main website.