Cisco IOS (Internetwork Operating System) is a proprietary operating system that runs on most Cisco Systems routers and switches.
The core function of Cisco IOS is to enable data communications between network nodes. In addition to routing and switching, Cisco IOS offers dozens of additional services that an administrator can use to improve the performance and security of network traffic. Such services include encryption, authentication, firewall capabilities, policy enforcement, deep packet inspection, Quality of Service, intelligent routing and proxy capability. In Cisco's Integrated Services Routers (ISRs), IOS can also support call processing and unified communications services.
There are three variants of the operating system: IOS XE, IOS XR and Nexus OS. IOS XE runs on enterprise-grade Cisco ISRs, Aggregation Services Routers and Catalyst switches. IOS XR runs on Cisco's service provider products, such as its Carrier Routing System routers. Nexus OS runs on Cisco's Nexus family of data center switches.