The Full Form of RADIUS is‍ Remote Authentication Dial-In User Service.

Remote Authentication Dial-In User Service (RADIUS) is a network protocol that provides security to networks against unauthorized access. RADIUS secures a network by enabling centralized authentication of dial-in users and authorizing their access to use a network service. It manages remote user authentication, authorization and accounting (AAA).

RADIUS is used by many companies to enable roaming between Internet service providers (ISPs), providing a single global set of credentials to be used on any public network. It is also used by independent or collaborating companies, which provide their own credentials to their own service users.

RADIUS is an open protocol distributed as a source code.

RADIUS was originally developed by the American corporation Livingston Enterprises in 1991. It is a network protocol for managing access server authentication and accounting as defined in Request for Comments (RFC) 2865, which was later moved into the Internet Engineering Task Force standards.

RADIUS supports maintaining company user profiles in a central database, where all remote servers connected to the central server are able to share the information. RADIUS is most widely used by Internet service providers and business enterprises because of its ubiquitous nature and broad support. It is used to authenticate access to internal and wireless networks and other integrated email services. These networks may use modems, virtual private network ports, Web servers, digital subscriber line (DSL), etc.

RADIUS performs three major functions:

• Authenticates users trying to establish a connection to a network
• Authorizes users to access requested network services
• Accounts for the use of those services