Authentication, Authorisation and Accounting (AAA) is the trio of services that control and track access to systems and networks. Authentication proves who you are, for example with a password, a certificate or a multi factor prompt. Authorisation decides what you can do once you are known, using roles, groups, policies and time or location rules so people get only the access they need. Accounting records what happened, such as when a session started and ended, which commands were run and how much data moved, so you can audit, bill or investigate. You see AAA in many places, from 802.1X on wired and wireless networks and VPN logins, to admin access on routers, switches and firewalls, and in cloud identity and access management. Common back ends are RADIUS for network access and TACACS Plus for device administration, often tied to a directory like Active Directory or LDAP. Good AAA needs accurate clocks for logs, clear naming for users and devices, and a plan for what to keep and for how long. The idea is to prove identity, grant the right level of access and keep a trustworthy record of what was done.
