The VPN (Virtual Private Network) allows users to connect from the Internet in their company. This connection is encrypted in a tunnel, so that data can not be recovered by a possible pirate.
The authentication procedure may be made either by e-certificate or by account / password. We must therefore choose the method that works for you.
The client connects to the VPN server, which authenticates with the Radius server (Remote Authentication Dial-In User Service) Central. After authentication, the client received his VPN company IP address, which allows access to company data, as if it were on the site.
Authentication: verification and control of User ID and Password in the Radius Client
This is a protocol used by machines to authenticate an user of a service or providing feedback on this user (traffic volume, connection time, ...). By extension, it is called Radius server or proxy server to tell server or proxy using the Radius protocol to communicate.
RADIUS uses UDP.
On Windows Server 2008R2 service Radius (IAS) was replaced by NPS (Network Policy Server).