freeradius and IP pools

Hi. I need to use freeradius in multiple ways. I mean: based on realm, I need to assign or not an IP address. For example: user@with_ip has to receive an IP from configured RADIPPOOL table user@without_ip has only to be authenticated (a user who log to a portal, for example). How can I make it possibile? Where can I setup this behaviuor? Thanks. Arrigo.