An authentication server is an application that facilitates authentication of an entity that attempts to access a network. Such an entity may be a human user or another server. An authentication server can reside in a dedicated computer, an Ethernet switch, an access point or a network access server.
Authentication is the process of determining whether someone or something is actually who or what it declares itself to be. When a potential subscriber accesses an authentication server, a username and password may be the only identifying data required. In a more sophisticated system called Kerberos, the subscriber must request and receive an encrypted security token that can be used to access a particular service. RADIUS (Remote Authentication Dial-In User Service) is a commonly used authentication method. TACACS+ (Terminal Access Controller Access Control System Plus) is similar to RADIUS but is used with Unix networks. RADIUS employs UDP (User Datagram Protocol) and TACACS+ employs TCP (Transmission Control Protocol.