GNS3 is a graphical network simulator that allows designing complex network topologies, running simulations and configuring devices from simple workstations to powerful Cisco routers.
GNS3 or Graphical Network Simulator-3 is a Graphical Network Emulator that allows the combination of virtual and real devices, used to simulate complex networks and perform Cisco CCNA/CCNP lab exercises. GNS3 is capable of emulating a number of network devices like Cisco routers and firewalls, Juniper routers, and frame-relay switches. It is based on VirtualBox, WinPCAP, PIX emulator Pemu/Qemu, VMware Dynamips, and Dynagen. It uses Dynamips emulation software to simulate Cisco Internetwork Operating Systems (IOS) and Virtual PC Simulator to simulate network hosts.