RADIUS, or Remote Authentication Dial-In User Service, is a widely deployed protocol that enables companies to authenticate, authorize and account for remote users who want access to a system or service from a central network server. Originally developed for dial-up remote access, RADIUS is now used by virtual private network (VPN) servers, wireless access points, authenticating Ethernet switches, Digital Subscriber Line (DSL) access, and other network access types. Extensible, easy to implement, supported, and actively developed, RADIUS is currently the de facto standard for remote authentication. RADIUS provides a complete, detailed guide to the underpinnings of the RADIUS protocol, with particular emphasis on the utility of user accounting. Author Jonathan Hassell draws from his extensive experience in Internet service provider operations to bring practical suggestions and advice for implementing RADIUS. He also provides instructions for using an open-source variation called FreeRADIUS.