Cisco IOS is an operating system by Cisco System which offer a better way to build a managable netwok interconnection, this operating system used on the vast majority of Cisco Router and Cisco Network Switch. As article i wrote in ubuntubuzz, here the list of Cisco IOS can be used for GNS3, Dynamips, or Dynagen :

Dear visitor,
We were sorry that we can't provide free download cisco ios images on this page (anymore), we afraid of breaking "Free Open Source Software" rule and violate some cisco authorities about legal software, very unfortunate that Cisco does not provide any flexibility to CCNA, CCNP, CCIE students to learn more about cisco Routers/Switch by provide a free IOS For Download. 


For complete instruction the way to use Cisco IOS for simulating a Cisco Router and Cisco Network Switch, see my previous article here.