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That's absolutely not how it works. Firstly, "www" is a subdomain of "example.com". The subdomain could match the hostname of the server that hosts it, but that's rarely the case (there can be multiple servers behind a load-balancer, for example). Secondly, you don't have to create a wildcard record for "example.com" to work. You just have to add an A record for it (just like you would do for any subdomain thereof).