You can assign a specific IP to each VPN Client, so that each time he/she establishes a VPN connection to your ISA server, he/she will receive the same IP Address.
1. Click Start, point to Administrative Tools , and then click Active Directory Users and Computers
2. Then go to the Organization Unit (OU) that contains the user you want to assign the same static IP each time he/she establishes a VPN Connection
3. Right Click the user, click on Properties
4. Go to the Dial-in tab for the user. Enable the checkbox beside Assign a Static IP Address, as shown in the following image
5. Enter the IP address you want to assign for this user. Once you enter it , click on OK.