The IOS has 2 main modes and many sub modes.
You move in and out of several different modes while configuring a router, and which mode you are in determines what commands you can use. Each mode has a set of commands available in that mode, and some of these commands are only available in that mode. In any mode, typing a question mark will display a list of the commands available in that mode.
Router>?
Unprivileged and privileged modes
When you first connect to the router , you enter the EXEC mode (or the Unprivileged mode). This is the most basic level of access to the Cisco IOS, the first mode in which you can issue commands.
Just type the question mark (Router > ? ) to display all the available commands at the EXEC level. You can also try some of the show commands to obtain information about the system. In unprivileged mode you can use basic commands like, show version to display the version of the IOS the router is running. Typing show ? at the prompt (Router>show ?), will display all the show commands available in the mode you are presently in.
You must enter privileged mode to configure the router, “enable” command is used for this. On a new router you will not be prompted for a password to enter the Privileged mode. It is HIGHLY recommended that you password protect the privileged mode. Once the privileged mode is password protected, when you issue the command “enable” it will prompt you for the password, to enter privileged mode.
To help you keep track of what mode you are in, the command line prompt changes each time you enter a different mode. When you switch from unprivileged mode to privileged mode, the prompt changes from:
Router>to
Router#
This is not a big deal when skipping between privileged and unprivileged mode, but there are many submodes you will use later on. This feature is very useful to keep track of where you are within the IOS and helps to avoid typing commands at the wrong place and getting annoying error messages !.
Within privileged mode there are many sub-modes. Once you enter privileged mode the prompt ends with a #. There are numerous modes you can enter, only after entering privileged mode. Each of these modes has a prompt of the form:
Router(submode)#
They still all end with the # sign. To get back to the unprivileged mode you type “disable” (very simple, enable to get to the privileged mode and disable to get back to the unprivileged mode)
Disclaimer
This document is to help you configure Cisco routers. We do not guarantee the accuracy of information in this document. Use at your own risk. The author will not be liable for any damages that occur as a result of using this document.