Assigning IP to incoming VPN connections

Greetings. I have a PIX 506E, and currently using MS dialup networking
(PPTP) for remote users to connect via VPN. The PPTP clients are
configured such that they will "obtain an IP automatically". I am
wondering if there is a way to control the IP of the remote clients. I
have tried to give a specific IP in the MS dialup networking (i.e.,
specify "Use the following IP address") but it cannot work. Is there
any way to do this?

Thanks for your help.

Regards,
Patrick

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