While Routing and Remote Access Service (RRAS) security is sufficient for small networks, larger companies often need a dedicated infrastructure for authentication.
IAS stores its authentication information in Active Directory, and can be managed with Remote Access Policies.
While IAS requires the use of an additional server component, it provides a number of advantages over the standard methods of RRAS authentication.
The initial version of Internet Authentication Service was included with the Windows NT 4.0 Option Pack.
Windows Server 2003's implementation introduces support for logging to a Microsoft SQL Server database, cross-forest authentication (for Active Directory user accounts in other Forests that the IAS server's Forest has a cross-forest trust relationship with, not to be confused with Domain trust which has been a feature in IAS since NT4), support for IEEE 802.1X port-based authentication, and other features.