If you have a few domains with different extensions and you need all of them to open the same website, you could have the website under one of them and forward the others. There are various methods to forward one domain name to another, including the so-called domain parking. If your website hosting plan permits it, though, it'll be better if you host each of the domains and create a URL redirect, not a domain redirect. The difference between the two is that while a domain name is hosted, you can still have content for it, set up subdomains, e-mail addresses, and so on., while with a parked domain it's not possible to do any of those things. For instance, if you are building localized Internet sites under various country-code domain names, you shall be able to work on them, but in the meantime, people will be forwarded to the main Internet site.