How can i change my cookie domain from the top level domain to a sub-domain.

Example: cookie for top level domain -> domain.com <- covers all sub-domains.

I only want cookie to cover -> www.domain.com <-- "www"


My objective is to create a cookie free domain -> static.domain.com to serve static contents from on the same server.

This is an experiment to improve performance of website.

Please show me which file(s) to change cookie domain