GNS3 is a Graphical Network Simulator.
The tool operates on popular desktop platforms such as Windows, Linux, Mac OS X.
It is designed for creation of different topologies on a computer network.
This function allows to perform tests requiring various network configurations.
The network simulator has an understandable user interface that enables its users to configure the virtual network. One can connect the virtual network to a real one.
GNS3 is widely utilized by those, who study for certifications like Cisco CCNA, CCNP, CCIE, etc. It is also used during software testing.