Cisco IOS has a Command Line Interface (CLI) and it has three command line modes. Each mode has access to different set of IOS commands.
User mode (User EXEC mode)
User Mode is the first mode a user has access to after logging into the router. The user mode can be identified by the > prompt following the router name. This mode allows the user to execute only the basic commands, such as those that show the system's status. The router cannot be configured or restarted from this mode.
The user mode can be identified as shown below
Privileged mode (Privileged EXEC Mode)
Privileged mode mode allows users to view the system configuration, restart the system, and enter router configuration mode. Privileged mode also allows all the commands that are available in user mode. Privileged mode can be identified by the # prompt following the router name. From the user mode, a user can change to Privileged mode, by running the "enable" command. Also we can keep a enable password or enable secret to restrict access to Privileged mode. An enable secret password uses stronger encryption when it is stored in the configuration file and it is more safe.
The Privileged mode can be identified as shown below
Global Configuration mode
Global Configuration mode mode allows users to modify the running system configuration. From the Privileged mode a user can move to configuration mode by running the "configure terminal" command from privileged mode. To exit configuration mode, the user can enter "end" command or press Ctrl-Z key combination.
The Global Configuration mode can be identified as shown below.
Global Configuration mode has various submodes, starting with global configuration mode, which can be identified by the (config)# prompt following the router name. Following are the important Global Configuration submodes.
• Interface mode (Router physical interface configuration mode)
• Subinterface mode (Router sub-interface configuration mode)
• Line mode (Router line configuration mode - console, vty etc.)
• Router configuration mode (Routing protocols configuration mode.)