Internet Authentication Service

Windows NT 4.0 Option Pack was Microsoft's first RADIUS implementation, compliant with the specifications of RFC 2138 and 2139. Now, Windows 2000 ships with both a Remote Authentication Dial-In User Service (RADIUS) client and a RADIUS server called Internet Authentication Service, or IAS.

RAS can behave like a RADIUS client and provide information to a RADIUS server that is used for authentication and accounting purposes. The RADIUS server can be either IAS or an external third-party server. Also, multiple servers can be configured to provide fault-tolerance. IAS as a RADIUS server validates client requests based on authentication and authorization, plus it provides accounting for users who connect using dial-up or VPN connections.