Cisco Internetwork Operating System (IOS) is a family of network operating systems used on many Cisco Systems routers and current Cisco network switches. Earlier, Cisco switches ran CatOS. IOS is a package of routing, switching, internetworking and telecommunications functions integrated into a multitasking operating system. Although the IOS code base includes a cooperative multitasking kernel, most IOS features have been ported to other kernels such as QNX and Linux for use in Cisco products.

IOS - Internetwork Operating System

Cisco 'Internetwork Operating System' is a multitasking, command based, configurable operating system that runs on Cisco routers and switches. It is a proprietary operating system developed by Cisco Systems of USA.

The IOS is usually stored in a flash memory and loaded on to the RAM during the boot process. The start-up configuration files are usually stored on a non-volatile RAM.

How to access the IOS on a Cisco device?
The simplest way to access the IOS is by connecting a PC to the console port of the device. Alternately, an operating system could be accessed remotely using an application layer protocol "Telnet".

An IOS operating system usually comes with 3 access modes: a user EXEC mode, a privileged EXEC mode and a global configuration mode. A user EXEC mode is basically used to view the basic information. A Privileged mode is used for advanced management, and the global configuration mode is used to configure a networking device.

A Cisco IOS has a complete control over the networking device hardware, on which it is installed. It provides a 'great' interface to a user to configure router & switching functions including: routing protocols, access lists, switching functions and security features.

The IOS network operating system was developed in the 1980s for routers that had only 256 kB memory and low CPU processing power. Through modular extensions IOS has been adapted to increasing hardware capabilities and new networking protocols. When IOS was developed, Cisco Systems' main product line were routers. The company acquired a number of young companies that focused on network switches, such as the inventor of the first Ethernet switch Kalpana, and as a result Cisco switches did not run the IOS. The Cisco Catalyst series would for some time run the CatOS.