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IOS is Cisco's internetwork operating system. Just as PCs have operating systems and LANs have network operating systems, it is Cisco's belief that internetworks need their own operating systems. Cisco's goal was to create an operating system that could evolve as the network evolved. IOS supports change and migration through its ability to integrate all evolving classes of network platforms, including routers, ATM switches, LAN and WAN switches, file servers, intelligent hubs, personal computers, and other devices.

IOS spans the core network, workgroups, remote access, and IBM internetworking. It supports formal and de facto standard interfaces, including major network protocols such as IP, IPX NetBIOS, SNA, and AppleTalk. IOS provides four internetwork services:

• Routing services    Locates optimal paths using adaptive routing techniques that reduce cost by using network bandwidth efficiently. Also supports multilayer switching and virtual LANs.


• WAN Optimization services    Supports all WAN services, including circuit-switched and packet-switched services.


• Management and security services    IOS provides an array of network management and security features, including configuration services, monitoring and diagnostics, encryption, authentication, accounting, and logging.


• Scalability services    IOS addresses key areas for avoiding congestion, overcoming protocol limitations, and bypassing obstacles that cause bottlenecks.

On many systems, IOS is accessed by connecting a terminal or PC to a console port on a networking device. A terminal emulation package or Telnet is used to access the IOS software. IOS has a user command interface with several command modes, including a user EXEC mode for viewing basic information, a privileged EXEC mode for advanced management, a global configuration mode for configuring system options, and several other modes of operation.

IOS has an extensive command set that requires some time and skill to master. Fortunately, a built-in help facility is available by typing a question mark on the command line. A complete set of documentation for Cisco IOS is available at the Web site listed on the related entries page.

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