GNS3 is a Graphical Network Simulator that allows emulation of complex networks. You may be familiar with VMWare, VirtualBox or Virtual PC that are used to emulate various operating systems in a virtual environment. These programs allow you to run operating systems such as Windows XP Professional or Ubuntu Linux in a virtual environment on your computer. GNS3 allows the same type of emulation using Cisco Internetwork Operating Systems. It allows you to run a Cisco IOS in a virtual environment on your computer. Dynamips is the core program that allows IOS emulation. GNS3 runs on top of Dynamips to create a more user friendly, graphical environment.
GNS3 also supports other emulation programs, namely Qemu, Pemu and VirtualBox. These softwares are used to emulate Cisco ASA and PIX firewalls, Cisco IPS, Juniper routers as well as hosts (Linux, Windows, Mac OS X, FreeBSD etc.) GNS3 makes all of this emulation magic work together and allow you, for instance, to have your Cisco router talking to your Linux host. The possibilities are almost endless!
GNS3 allows the emulation of Cisco IOSs on your Windows, Linux and Mac OS X based computer. Emulation is possible for a long list of router platforms and other devices. Using an EtherSwitch card in a router, switching platforms may also be emulated to the degree of the card’s supported functionality. This means that GNS3 is an invaluable tool for preparing for Cisco certifications such as CCNA, CCNP and CCIE.