If no configuration file is found, router presents you with a system configuration dialog. After the configuration file is found and loaded into RAM, router reads and executes commands from configuration file one at a time. These process enables interfaces and sets parameters related to protocols, including routing protocols. Exec is the command interpreter of the IOS . The set of commands the you can enter depends on the level of access.
User mode is the default mode when the router boots up.
Privileged mode provides all the functions of user mode and also provides access to configuration mode. To exit from Privileged mode, type disable . Configuration mode is accessible only from the privileged mode. To enter configuration mode, enter config terminal from the Privileged mode. For example to configure Ethernet interface you need to type interface ethernet0 from the Configuration mode.