After creating enterprise certificate authority, the fourth step in setting up a Windows-based virtual private
network is to install IAS.
Step 4: Install IAS
The IAS server will authenticate users who enter your corporate network through the VPN connection. As such, your IAS server must be a member server in one of your domains and must be running Windows Server 2003.
Get step-by-step instructions on installing IAS on your customer's VPN.
Get more info on setting up your customer's VPN.