Cisco IOS

Cisco Internetwork Operating System or IOS is the software used to run Cisco routers, switches and firewalls as well as other devices. Cisco certifications are considered amongst the hardest in the IT industry because most configurations are done via the command line interface or CLI.

When you purchase a new or used Cisco router or switch it will usually come with a version of IOS installed. For the Cisco CCNA exam you will want to run a fairly new version of IOS if possible. Don’t worry if your router is running 12.1 release (12.4 is the latest). So long as you can run the commands most commonly used for the exam you will be fine. For the CCNP and CCIE you may need to find the latest release of IOS.

You must buy a version of Cisco IOS if you have none to use on your own equipment. Of course there are many sites offering illegal copies of Cisco IOS but you will find many to have had viruses inserted. Many companies have support contracts with Cisco which include access to Cisco IOS’s so if you work for one of these you may be able to get access to one.

To load an IOS onto your router please watch our videos.

IOS Naming

Cisco IOS all have numbers and letters associated with them so you can identify one from another. When considering an IOS release for home lab use you need to determine the model of router or switch you need an image for, the release version of IOS and the features it offers.

If you issue the ‘show flash:’ or ‘dir flash:’ command on your router you will see the version installed:

RouterA#show flash
System flash directory:
File Length Name/status
1 14692012 c2500-js-l.121-17.bin
[14692076 bytes used, 2085140 available, 16777216 total]
16384K bytes of processor board System flash (Read ONLY)

c2500 – image works on the 2500 series router

js – the image is enterprise feature set and the s stands for plus.

121-17 – the release is 12.1 on the 17th maintenance version.

.bin – a binary file which is what all Cisco IOS come in.

For your home lab you need not worry too much about feature sets. The more features an IOS can support the bigger it is and larger companies will need more features such as advanced encryption. All you need is access to routing protocols, DHCP, DNS and other basic services.

Image Size

A mistake many CCNA level students make is to try to load an image onto their router which will not fit. If you issue a show version command on your router you will see how big your flash memory is:

Router#show version
Cisco Internetwork Operating System Software
IOS ™ 2600 Software (C2600-IS-L), Version 12.2(4)T1, RELEASE SOFTWARE
Copyright (c) 1986-2001 by cisco Systems, Inc.
ROM: System Bootstrap, Version 11.0(10c), SOFTWARE ⇐ ROM code
BOOTLDR: 3000 Bootstrap Software (IGS-BOOT-R), Version 11.0(10c)
System image file is “flash:c2600-is-l_122-4_T1.bin” ⇐ Flash image
cisco 2522 (68030) processor ⇐ CPU
with 14336K/2048K bytes of memory. ⇐ DRAM
Processor board ID 18086064, with hardware revision 00000003
32K bytes of non-volatile configuration memory. ⇐ NVRAM
16384K bytes of processor System flash (Read ONLY) ⇐ EEPROM/FLASH


The flash in the above router is 16 meg. If you try to load the shown image on it will try to and then fail leaving you with no image at all. You will then be forced to do a rommon recovery.






If you find that certain commands are not available on your router or switch then you need to look at your release level or the feature set. Check Google for the command you want to enter and you will usually see a reference to it on Ciscos web site plus the IOS release it started in.








Cisco feature navigator

Cisco IOS tutorial

How to choose a Cisco IOS release

468 ad


  1. Kurt muehlbauer

    Does each router have to have an ios? Or if you find one that has one installed will that work? If they both have to have one must they be same version?


    • paulwbrowning

      Yes, each router needs an IOS but you can copy it across if it is the same model. Check out IOS upgrades videos on YouTube.



Leave a Reply