If you have several domains with different extensions and you would like all of them to open precisely the same website, you may have the site under one of them and forward all the rest. There are a number of approaches to forward one domain to another, like the so-called domain parking. When your website hosting plan permits it, though, it'll be better if you host all the domains and create a URL redirect, not a domain redirect. The main difference between the two is that while a domain is hosted, you could still have content for it, create subdomains, email addresses, etc., while with a parked domain it's not possible to do any of these things. For instance, if you're building localized sites under different country-code domains, you'll be able to work on them, but in the meantime, people shall be forwarded to the primary site.