The Cisco IOS is the most important part of the router. The IOS is the program code that defines how the router functions. Without it, the router cannot route packets. The IOS is feature-rich and pretty much standard across the various Cisco platforms. So once you become familiar with IOS commands on one router, you do not have to learn new commands to work on a new router.The IOS provides a labor-saving command line interface for configuring routers that is easy to navigate.
User Interface
The Cisco IOS provides a robust user interface called EXEC for its routers. EXEC, short for executive, intercepts commands and executes them. For security purposes, EXEC supports two types of access:
_ User mode
_ Privileged mode
User Mode
Upon logging in to the router, you are automatically put into user mode. EXE commands in user mode allow you to display information but you cannot change router configuration settings. User mode commands are a subset of the larger privileged mode commands.
Privileged Mode
You must enter a password before you can access privileged mode. Privileged mode allows execution of all of the user mode commands, as well as setting configuration parameters, performing extensive testing and debugging, and accessing the other router modes.