GNS3 is a graphical network simulator that permits you to design complex network topologies.
Unlike the Cisco Packet Tracer, GNS3 provides more functionalities, characteristics, and a real-life virtual environment.
GNS3 supports several types of routers and switches, so it can help you study for certification exams such as Cisco CCNA, CCNP, CCIP, CCSP, CCVP and CCIE, Juniper JNCIA, JNCIS or JNCIE while being equally useful to network engineers or admins.
GNS3 offers an easy way to design and build networks of any size without the need for hardware. And the best part is it’s free!
It is based on Dynamips, Pemu/Qemu, and Dynagen.