Cisco IOS Explained with Features and Functions

This tutorial explains what the Cisco IOS is and what are the main responsibilities of the IOS. Learn the features and functions of the Cisco IOS in detail.

What is the Cisco IOS?

The Cisco IOS (Internetwork Operating System) is a proprietary operating system that provides routing, switching, and internetworking features. It controls and manages the hardware on which it runs. Technically, it provides an interface between a user and the hardware, allowing the user to execute commands to configure and manage the Cisco device.

Originally, the IOS was developed for Cisco routers, but a few years later Cisco decided to use the IOS to its other platforms, including the Catalyst switches. Currently, the IOS runs on most Cisco routers as well as a growing number of Cisco Catalyst switches such as Catalyst 2960 and 3560 series switches.

Functions of the IOS

The IOS is responsible for the following functions: -

  • To carry network protocols and functions
  • To connect between different data link layer technologies
  • To connect high-speed traffic between devices
  • To secure network resources
  • To control unauthorized access
  • To provide scalability for ease of network growth
  • To keep the network stable and reliable

Let’s understand these functions in detail.


Routers and switches are manufactured in two types of hardware platforms: fixed chassis and modular chassis.

If a router or switch is built on the fixed chassis platform, then we have to use it as it is. We cannot add any additional ports or interfaces to it and at the same time, we cannot remove any existing interfaces or ports.

The following image shows a fixed chassis router.

fixed chassis platform router

But if a router or switch is built on the modular chassis platform, then we can install the interfaces or ports of our choice. A modular router or switch has few empty slots along with the fixed ports and interfaces. In empty slots, we can install interfaces or ports of our choice.

The following image shows a modular chassis router and modular interfaces.

modular chassis router

The modular chassis platform provides more flexibility but costs more than the fixed chassis platform.

Cisco creates separate versions of the IOS for both platforms. If you purchase a fixed chassis platform, then you will get an IOS that does not contain the features that are required for the modular chassis platform. Since Cisco uses customized versions of the IOS, you never need to pay for the features that you do not need.


Each media type uses a separate format to transfer data over it. The IOS not only understands the formats of almost all modern media types but can also convert them.

You can use an IOS running router to connect two different networks that use different media types such as: -

  • To connect a LAN network to a WAN network
  • To connect a wireless network to a wired network.


The first IOS was written in 1986 by William Yeager. Since then, to keep the IOS up-to-date, to ensure critical resources always remain accessible, and to adopt new technologies introduced in the market, Cisco has not only tweaked and tuned IOS several times but also has added new features in every update.

Because of the success of its IOS software, Cisco has moved from a garage-router company to one of the world's largest companies in less than two decades. Nowadays a large portion of the Internet backbone is composed of Cisco products. Due to the reliability and stability of the IOS, most enterprise networks, as well as ISPs use Cisco products in one form or another.


The IOS includes a wide range of security features that allow you to strictly control your network resources and networking devices according to your internal security policies. You can configure the IOS to allow or deny a specific host or a range of hosts. You can also configure the IOS to allow or deny access to a particular application.

Once security policies, commonly known as the access control lists, are configured the IOS actively monitors all traffic that passes through it and allows or denies the traffic based on the configured rules. For example, if the IOS receives traffic from a host that is blocked in the access list, the IOS discards the traffic immediately. Or if the IOS receives traffic from a host that is allowed in the access list, the IOS process the traffic and forwards that to the destination.

That’s all for this part. In the next part of this tutorial, we will understand the Cisco IOS naming convention in detail. If you like this tutorial, please don’t forget to share it with friends through your favorite social network.

ComputerNetworkingNotes CCNA Study Guide Cisco IOS Explained with Features and Functions