GNS3 is the name of software which is provided by the Cisco Organization. If you want to download this software then link is given above. If you want to work on actual operating system on behalf of IOS internetworking operating system then can install and configure GNS3.
If you are giving international exam for Cisco Certification like CCNA, CCNP and CCIE, then can practice all the cisco lab which is related to the CCNA Practical, CCNP Practical, CCIE Practical then easily can practice for the same. CCNA Practical will be related to the routing and switching, CCNP Practical also can practice for router and switches along with the firewall operations.
GNS3 will work on behalf of actual internetworking operating system, if you want to interest your networking devices, switch, router and firewall then can easily interact with internal as well as the external networking which is connected to the outside and inside.
GNS3 will support all the latest technology and latest configuration of the command line interface and graphical user interface.