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.
There are a number of router simulators on the market, but they are limited to the commands that the developer chooses to include. Almost always there are commands or parameters that are not supported when working on a practice lab. In these simulators you are only seeing a representation of the output of a simulated router. The accuracy of that representation is only as good as the developer makes it. With GNS3 you are running an actual Cisco IOS, so you will see exactly what the IOS produces and will have access to any command or parameter supported by the IOS.
In addition, GNS3 is an open source, free program for you to use. However, due to licensing restrictions, you will have to provide your own Cisco IOSs to use with GNS3. Also, GNS3 will provide around 1,000 packets per second throughput in a virtual environment. A normal router will provide a hundred to a thousand times greater throughput. GNS3 does not take the place of a real router, but is meant to be a tool for learning and testing in a lab environment. Using GNS3 in any other way would be considered improper.