1. Remote Authentication Dial-In user Service. A standard for authentication and accounting, RADIUS is primarily used to control dial-up access to PPP and other services. The protocal was standardized in RFC 2058, the current implementation is defined in RFCs 2138 and 2139. RADIUS uses UDP packets, older servers use ports 1645 and 1646, the current standard is port 1812 for authentication and 1813 for accounting. From Linux Guide @FirstLinux