frammed ip adress

Hi, in my working solution, i have pptp (vpn) configured with radius using LDAP. Each user has a value Framed IP Adress which will assign him exact IP adress. Currently i am rebuilding ldap structure to groups. And i want the users which will be members of group foo , to have dynamically assignet ip adresses from pool 10.123.40.0/255.255.255.0 . How can i achieve ? Which radius attributes should i use? Thanks a lot!