GNS3 is a network simulator that allows you to create and configure virtual networks. It is free and open source software released under the GNU General Public License (GPL).
GNS3 is used by many professionals in the networking industry and academia for simulating network environments without the need for physical hardware. It can be used to simulate various scenarios, such as Cisco IOS, Juniper JUNOS or Quagga router configurations.
While GNS3 is free to download and use, it does require some additional components in order to function properly. These components, such as IOU (IOS on Unix), QEMU (Quick Emulator) or Dynamips, must be downloaded separately.
In addition, GNS3 offers a paid subscription service called GNS3 Cloud which gives users access to additional features, such as cloud storage of projects and enhanced support.