Operating Systems

All end devices and network devices require an operating system (OS). As shown in Figure 1, the portion of the OS that interacts directly with computer hardware is known as the kernel. The portion that interfaces with applications and the user is known as the shell. The user can interact with the shell using a command-line interface (CLI) or a graphical user interface (GUI).
When using a CLI as shown in Figure 2, the user interacts directly with the system in a text-based environment by entering commands on the keyboard at a command prompt. The system executes the command, often providing textual output. The CLI requires very little overhead to operate. However, it does require that the user have knowledge of the underlying structure that controls the system.
A GUI interface such as Windows, OS X, Apple iOS, or Android allows the user to interact with the system using an environment of graphical icons, menus, and windows. The GUI example in Figure 3 is more user-friendly and requires less knowledge of the underlying command structure that controls the system. For this reason, many individuals rely on GUI environments.
However, GUIs may not always be able to provide all of the features available at the CLI. GUIs can also fail, crash, or simply not operate as specified. For these reasons, network devices are typically accessed through a CLI. The CLI is less resource intensive and very stable when compared to a GUI.
The network operating system used on Cisco devices is called the Cisco Internetwork Operating System (IOS). Cisco IOS is used for most Cisco devices regardless of the type or size of the device.
Note: The operating system on home routers is usually called firmware. The most common method for configuring a home router is by using a web browser-based GUI.

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