If your are premium user at wordpress.com, you can do the domain mapping. Here is what you need to do: Go to your Client Area > My Domain > Details and change the nameservers to the following, being sure to remove any existing nameservers that may be there:
Once you've told one particular DNS server about your new domain, it will have to transfer that information to all the other servers on the internet, which could take a couple of days for the new pointer to propogate. So be aware that your new domain won't necessarily be working instantly.
Go to Settings -> Domains in your Dashboard. If there is a Put blog here link in the same row as your domain name, click it to set your domain name as the default for all of the blog’s URLs
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