GNS3

What GNS3 is

GNS3 (Graphical Network Simulator 3) is a free, open source platform for building network labs with real network operating systems. You draw a topology in the GNS3 GUI or web UI, and the GNS3 server starts each device and wires them together. According to its project documentation, the server manages emulators such as Dynamips, VirtualBox and QEMU/KVM, with Docker supported on Linux for container-based nodes. Clients such as the desktop GUI and the web UI control the server through an HTTP REST API.

Simulator or emulator: why it matters

Packet Tracer simulates Cisco devices: it imitates their behaviour, and only a subset of commands exist. GNS3 emulates: it runs the real software images, such as Cisco IOSv, Arista vEOS, Juniper images, Linux hosts and virtual firewalls. Your lab therefore behaves like the equipment you will meet at work, including features and bugs a simulator leaves out. That is what you want for BGP, multi-vendor labs, firewalls, VPNs and testing automation scripts.

What you need

• The software: the GNS3 GUI on your computer plus the GNS3 VM, a virtual machine that runs the heavier images, or a separate Linux server.
• Memory and CPU: real images use real resources. A few routers run on a typical laptop; large labs need more RAM or a server.
• Images: GNS3 is free, but vendor images are not part of it. You need images you are licensed to use, or ones offered free by the vendor, such as the images included with Cisco Modeling Labs Free or vendors' free lab editions. Do not download images from unofficial sites.

Is it worth it?

Once you have the basics from Packet Tracer, yes. GNS3 is the standard free lab platform for network engineers, used for certification study beyond the associate level, for testing designs before changing production and for trying automation safely. Its appliance library gives templates for many vendors' images, and the community forum is active.

The cost is setup time. Expect an afternoon to install the GUI and the VM and get your first images running, and some troubleshooting of virtualisation settings on your computer.

Versions

The stable line is 2.2 (release 2.2.61 in July 2026 on GitHub), and version 3 is in development, with pre-releases published alongside. Follow the documentation for the version you install, because screens and steps differ between them.

Common questions

Do I still need Packet Tracer? Many learners use both: Packet Tracer for quick CCNA labs, GNS3 when they need real software.

Can I capture traffic? Yes. You can start a Wireshark capture on any link in the topology, which is one of its most useful features.

Is it only for Cisco? No. It is multi-vendor, which is one of its main advantages.