Windows Authentication for several sites in one domain

Hi!
I have two different web sites in Intranet zone. They are placed in one domain (like http://site1 and http://site2.site1). They use Windows Authentication for common domain (like mycorp\User1, mycorp\User2, mycorp\User3).

First web site(http://site1) have iframe element with hyperlink to second web site(http://site2.site1). When user loads first web site he needs write login and password twice. How to make that http://site1 and http://site2.site1 ask authentication only once?
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