AAA stands for Authentication, Authorization and Accounting. + Authentication: Specify who you are (usually via login username & password) + Authorization: Specify what actions you can do, what resource you can access + Accounting: Monitor what you do, how long you do it (can be used for billing and auditing) An example of AAA is shown below: + Authentication: "I am a normal user. My username/password is user_tom/learnforever" + Authorization: "user_tom can access LearnCCNA server via HTTP and FTP" + Accounting: "user_tom accessed LearnCCNA server for 2 hours". This user only uses "show" commands.