VPN

I used pptp vpn for dial-in from internet, i want different user can access diferent service, so i need give different user different ip(s), because every user may be dial-in over 1 times, so need assign 1 range of ip to every user. while i can specify a ip for a user in pptp users, but specify one ip only for a user. How do i do? anyone could give me some help? thanks a lot!
PS: I'm sorry my english is very poor, hehe...

BTW: I send a message to mpd's developer Alexander Motin at sourceforge.net use http://sourceforge.net/sendmessage.php?touser=892211, but i don't know how to receive message reply from Alexander Motin, if anyone know how to do, tell me please too, thanks!