freeradius with pptp

Hi everyone, I have installed freeradius(mysql module) vpn(pptp) servers. The vpn users are authenticated by username and password in radcheck table, but not specified a IP address. In my servers, there are multiple IP address. When use vpn, my computer shows a constant IP with all vpn accounts. How can i specify IP for vpn accounts? What i want is that different vpn users use different IP address. Could anyone help me? -- Best regards,