GNS3 is a graphical network simulator that allows simulation of complex networks.
To allow complete simulations, GNS3 is strongly linked with : Dynamips, the core program that allows Cisco IOS emulation. Dynagen, a text-based front-end for Dynamips. Pemu, a Cisco PIX firewall emulator based on Qemu.
GNS3 is an excellent complementary tool to real labs for Cisco network engineers, administrators and people wanting to pass certifications such as CCNA, CCNP, CCIP or CCIE. It can also be used to experiment features of Cisco IOS or to check configurations that need to be deployed later on real routers. This project is an open source, free program that may be used on multiple operating systems, including Windows, Linux, and MacOS X.