If you have several domains with different extensions and you would like all of them to open the exact same site, you could have the site under one of them and redirect all the rest. There are several ways to forward one domain name to another, such as the so-called domain parking. If your hosting plan allows it, though, it shall be better if you host all 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, set up subdomains, e-mail addresses, etc., while with a parked domain you cannot do any of those things. For instance, if you're building localized websites under various country-code Internet domain names, you'll be able to work on them, but in the meantime, visitors shall be redirected to the primary Internet site.