Cisco IOS (originally Internetwork Operating System) is the software used on the vast majority of Cisco Systems routers and current Cisco network switches. (Earlier switches ran CatOS). IOS is a package of routing, switching, internetworking and telecommunications functions tightly integrated with a multitasking operating system. The first IOS was written by William Yeager.

Cisco IOS has a characteristic command line interface (CLI), whose style has been widely copied by other networking products. The IOS CLI provides a fixed set of multiple-word commands — the set available is determined by the "mode" and the privilege level of the current user. "Global configuration mode" provides commands to change the system's configuration, and "interface configuration mode" provides commands to change the configuration of a specific interface. All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege. Through the CLI, the commands available to each privilege level can be defined.

Versioning

Cisco IOS is versioned using three numbers and some letters, in the general form a.b(c.d)e, where:

• a is the major version number.
• b is the minor version number.
• c is the release number, which begins at one and increments as new releases in the same a.b train are released.
• d (omitted from general releases) is the interim build number.
• e (zero, one or two letters) is the release train identifier, such as none (which designates the mainline, see below), T (for Technology), E (for Enterprise), S (for Service provider), XA as a special functionality train, XB as a different special functionality train, etc.

For example, release 12.3(1) is the first mainline Cisco IOS release of version 12.3. 12.3(2) is the next release, and so on. 12.3(1)T is the first release of the T train, 12.3(2)T the next, and so on. Interim builds are candidates for the next release, and are frequently made available by Cisco support as a faster way to provide fixes for bugs before the next release is available. For example, 12.3(1.2)T is the 2nd interim build after release 12.3(1)T.

Rebuilds - Often a rebuild is compiled to fix a single specific problem or vulnerability for a given IOS version. For example, 12.1(8)E14 is a Rebuild, the 14 denoting the 14th rebuild of 12.1(8)E. Rebuilds are produced to either quickly repair a defect, or to satisfy customers who do not want to upgrade to a later major revision because they may be running critical infrastructure on their devices, and hence prefer to minimise change and risk.

Interim releases - Are usually produced on a weekly basis, and form a roll-up of current development effort. The Cisco advisory web site may list more than one possible interim to fix an associated issue (the reason for this is unknown to the general public).