what ports should be open between a domain's site's subnets

[gwu]

i am creating a couple of vlans/subnets that will serve one site in a domain.

1)What ports need to be open between the two. I need to restric file sharing so one subnet can never share files between another.

2)what is the purpouse of creating multiple subnets in a site in the "sites and services" snap-in? I created the different vlans/subnets using the cisco switch and the users on the one vlan were able to connect/file share with the dc on the other lan(the firewall has not restricted the ports yet) without creating the "subnet" in "sites and services".

thanks