DHCP server option

Hi I am using a Linux based DHCP server in my LAN and users are restricted to get a specific address on the basis of their MAC address. Now I just configured a windows based Incoming PPTP VPN connection and gave a range of address in the DHCP. Now the problem is that my users in the LAN are getting their IP addresses from this windows based DHCP server. How can I restrict a Client to get address from a specific DHCP server.

Note: The windows based DHCP server is also the Domain Controller for my LAN so I cant put it on a seperate VLAN.

Any help pls waiting…

Regards,
Ubaid ur Rehman
Descon Chemicals Pvt Ltd