GNS3 is a handy and accessible piece of software that was designed to be a graphical network simulator. It was specially developed in order to allow you to design complex network topologies and to launch simulations on them. You can also download GNS3 Portable. Features Emulating Cisco routers. Simulating simple Ethernet, ATM and Frame Relay switches. Load-balancing between multiple hypervisors. Load and save in Dynagen's INI-like format. Image export (JPEG, PNG, BMP and XPM).