Configuring RAS for a VPN connection.

In the RAS Services properties, clicked on Network. For the TCP/IP configuration, I need help. We do not have a DHCP server set up here. The client machine that needs to VPN here is on a cable modem and has an IP address assigned by his ISP. How do I configure the RAS properties IP address for the client? Anything I assign will be on a different network than the ISP assigned — so can that client have 2 IP addresses? 1 assigned by the cable modem ISP, and 1 assigned by our VPN server? Whenever he tries to VPN, an error occurs. (I guess the question boils down to this: How do I configure RAS for a VPN connection so a user with a cable modem can connect via VPN?)