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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.

Not all Cisco products run IOS. Notable exceptions include ASA security products, which run a Linux-derived operating system, carrier routers which run IOS-XR and Cisco's Nexus switch and FC switch products which run Cisco NX-OS.

Source: https://en.wikipedia.org/wiki/Cisco_IOS