In case you have a lot of domains with different extensions and you need all of them to open the same site, you could have the site under one of them and redirect all the rest. There are various ways to forward one domain name to another, like the so-called domain parking. When your hosting package allows it, though, it'll be better if you host all of the domains and create a URL redirect, not a domain redirect. The main difference between the two is that while a domain address is hosted, you could still have content for it, set up subdomains, email addresses, and so forth., while with a parked domain you cannot do any of these things. As an example, if you're building localized websites under several country-code domains, you'll be able to work on them, but meanwhile, visitors shall be forwarded to the primary site.