domain mapping eg site.example.com to site.com

domain mapping eg site.example.com to site.com

I have a website running with considerable good traffic lets say http://www.site.com, I have installed a new wordpress website lets say http://www.example.com. I wants to create a subdomain eg site.example.com that will then lead traffics to http://www.site.com and viceversa. How can i achieve this?