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:
ns1.wordpress.com
ns2.wordpress.com
ns3.wordpress.com
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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