GNS3 is a fantastic piece of software, it glues together different open source software and allows to emulate a network that includes Cisco routers (using real Cisco firmware), Cisco switches (using IOU, Cisco IOS on Unix), Cisco ASA and any other devices that can run on Qemu or Virtual Box emulator.
It also allows to connect the virtual network to the physical network, it is possible to access Internet in the emulated world and vice-versa. GNS3 is available on Windows, Mac OSX and on Linux, but it shines on Linux because, to use IOU, if you are not on Linux you need a Linux virtual machine running on VirtualBox.… Read the rest