RADIUS (Remote Authentication Dial-In User Service) is a client-server protocol that provides centralized authentication, authorization, and accounting management for users who connect and use a network service. RADIUS is an AAA (authentication, authorization, and accounting) protocol that manages network access. It uses two types of packets to manage the full AAA process: Access-Request, which manages authentication and authorization; and Accounting-Request, which manages accounting. RADIUS servers check the users information against a locally stored flat file database or a user database. RADIUS authentication works by verifying the users proof of identification, along with other information related to the request, such as the users network address or phone number, account status, and specific network service access privileges. The RADIUS server checks that the information is correct using authentication schemes such as PAP, CHAP, or EAP. If the credentials are correct, the server authenticates the user and enables the remote user to connect to the network. RADIUS is commonly used for remote access across different types of networks, including wireless networks, Ethernet networks, and other types of remote user access through the internet.