User EXEC mode commands
Initially, a user logs into the User Exec mode. This is the mode with the least number of commands. You can get a list of all available commands by typing the character “?”.
As you can see, most of the commands available are used to show statistics and perform some basic troubleshooting. The prompt on the left side of the screen always displays the device hostname (R1 in this case), followed by the character >.
All commands can be abbreviated to their first letters of the command name. For example, you can abbreviate ping by typing pin, because no other command in the User EXEC mode IOS mode begins with these letters.
Privileged EXEC mode commands
This IOS mode is also called enable mode because you must enter the enable command from a user EXEC mode, if you want to access this mode. You can use more commands in the privileged EXEC mode than you were able to use in the user EXEC mode. You can save a device configuration or reload a device in this mode. You can also enter a third mode, the configuration mode.
An access to this mode can be protected by a password.
The prompt for this mode shows # after the device hostname.
Global configuration mode commands
To change a device configuration, you need to enter the global configuration mode. This mode can be accessed by typing configure terminal (or conf t, the abbreviated version of the command) from the enable mode. The prompt for this mode is hostname(config).
Global configuration mode commands are used to configure a device. You can set a hostname, configure authentication, set an IP address for an interface, etc. From this mode you can also access submodes, for example the interface mode, from where you can configure interface options.
You can get back to a privileged EXEC mode by typing the end command. You can also type CTRL + C to exit the configuration mode.
A global configuration mode contains many submodes. For example, if you want to configure an interface you have to enter that interface configuration mode. Each submode contains only commands that pertain to the resource that is being configured.
To enter the interface configuration mode you need to specify which interface you would like to configure. This is done by using the interface INTERFACE_TYPE/INTERFACE_NUMBER global configuration command, where INTERFACE_TYPE represents the type of an interface (Ethernet, FastEthernet, Serial…) and INTERFACE_NUMBER represents the interface number, since CIsco devices usually have more than one physical interface. Once inside the interface configuration mode, you can get a list of avaliable commands by typing the “?” character. Each submode has its own prompt. Notice how the command prompt was changed to HOSTNAME(config-if).