its a server which is used to authenticate clients using the RADIUS (Remote Authentication Dial In User Service) client/server protocol. it uses a AAA (Authentication, Authorization, Accounting) concept. RADIUS servers are used by many companies, organizations, universities and especially ISPs. When you dial in to the ISP you must enter your username and password. This information is passed to a RADIUS server, which checks that the information is correct, and then authorizes access to the ISP system. it uses UDP ports 1812 for RADIUS Authentication and 1813 for RADIUS Accounting.