A device used in network access control. It stores the usernames and passwords that identify the clients logging in, or it may hold the algorithms for token access (see authentication token). For access to specific network resources, the server may itself store user permissions and company policies or provide access to directories that contain the information.

RADIUS is the most widely used protocol for authentication servers. TACACS+ is a Cisco-developed product that has also been popular. The authentication server may be a stand-alone system or software that resides in an Ethernet switch, wireless access point (AP) or network access server (NAS). See AAA server, RADIUS and 802.1x.