GNS3 is a graphical network simulator that allows you to easily design network
topologies and then run simulations on them. At the moment GNS3 supports IOS
routers, ATM/Frame Relay/Ethernet switches and hubs. You can even extend your own
network by connecting it to your virtual topology.

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.

To do his magic, GNS3 is based on Dynamips and in part on Dynagen, it has been
developed in python and through PyQt the GUI part is made with the powerful Qt library,
famous for its use in the KDE project. GNS3 also uses the SVG technology (Scalable
Vector Graphics) to provide high quality symbols for designing your network topologies.