GNS3 is software that lets you build a complete computer network on your own computer, without buying a single router or switch. The name stands for Graphical Network Simulator. The project started in 2008. It is now developed by GNS3 Technologies with the help of an active community, and its code is released under the free GPL v3 license. It has become a fixture of networking courses, especially among people preparing for Cisco certifications.
What GNS3 is for
The first thing to understand is what sets GNS3 apart from a simple simulator. A tool like Packet Tracer, made by Cisco, imitates the way a router behaves: it reproduces the most common commands, but it is not the real system. GNS3 does emulation instead. It runs the actual software of the device, the same one that runs on a router sitting in a rack. The commands, the protocols and even the bugs are therefore the real ones.
In practice, you drop devices onto a workspace, connect them by drawing cables with the mouse, then start them up. Each device opens in a console where you type exactly the same commands you would on physical hardware. To run these systems, GNS3 relies on several engines. Dynamips handles older Cisco IOS images. QEMU runs more recent systems, such as the virtual routers from Cisco, Juniper or Fortinet. VirtualBox, VMware and Docker let you add client machines or servers. A right-click on a link launches Wireshark, which shows the packets travelling across it. It is one of the most popular features for understanding what is really happening on a network.
The software comes in two parts: the graphical interface, installed on your computer, and a server that does the heavy lifting. On Windows and macOS, that server usually runs inside the GNS3 VM, a virtual machine supplied by the publisher, because emulation works much better on Linux. The server can also be installed on a remote machine, so a whole team can work on the same lab. Finally, a catalog of appliances provides ready-made device templates that you can import in a few minutes.
Who is it for?
GNS3 is aimed first at students and at candidates for the CCNA or CCNP certifications, who can practice as much as they want without any hardware. Trainers use it to prepare lab exercises. Network administrators rely on it to test a delicate change, such as migrating a routing protocol, before carrying it out in production.
The tool does have its demands. Every virtual device uses memory and processor time, and a lab with a dozen modern routers calls for a well-equipped computer. Vendor images are not included: you have to obtain them legally, for example through a Cisco Modeling Labs subscription or from your own equipment. The first setup, with the virtual machine and the images, also takes some patience. A complete beginner will be more comfortable with Packet Tracer, which is easier to pick up. For a multi-vendor lab you can use from a web browser, EVE-NG is the other big name in the field.
Download GNS3 safely
The official version is available on gns3.com. The site offers a complete installer for Windows, a package for macOS and instructions for the main Linux distributions, along with the GNS3 VM for VMware, VirtualBox or Hyper-V. The source code is published on GitHub. Downloading requires a free account on the publisher’s website.
The software itself has no paid edition. The publisher earns its money mainly from video courses sold on its GNS3 Academy platform, which nobody is required to buy. A website that asks you to pay to install GNS3, or that offers an installer hosted somewhere else, is therefore not a trustworthy source. The same caution applies to router images shared on forums, which are often passed around without a license.
Frequently asked questions
Is GNS3 free?
Yes. GNS3 is free software released under the GPL v3 license, with no paid version. Some of the device images you run inside it, however, remain subject to their vendor’s license.
Does GNS3 work on Windows and Mac?
Yes, there is a version for Windows, one for macOS and one for Linux. On Windows and Mac, it is recommended to install the GNS3 VM as well, so that devices run in good conditions.
What is the difference between GNS3 and Packet Tracer?
Packet Tracer simulates the behavior of Cisco devices, which makes it simple but limited. GNS3 runs the real systems of several vendors. It is more realistic and more complete, but also more demanding in terms of resources and setup.
