The Remote Authentication Dial-In User Service (RADIUS) is an application layer client-server networking protocol. A RADIUS Server and RADIUS Clients are used in the RADIUS protocol. A RADIUS Client (or Network Access Server) is a networking device that authenticates users (such as a VPN concentrator, router, or switch). RADIUS servers are also used to keep track of expenses. RADIUS accounting gathers information for network monitoring, billing, and statistics.