Cisco IOS, Internetworking Operating System, is a very specialized proprietary operating system (OS) only for network routers and switches. It is dynamically upgradable, internetwork optimized, router-centric; provides common IP fabric, functionality and command-line interface (CLI) across a network. In its niche it is viewed as sophisticated, cost-effective. Does not run applications in the normal sense. Extremely command line oriented, said to be fully configured via only one configuration file.