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GNS3 (Graphical Network Simulator-3) is a graphical network emulator that is free to use, open source and reliable. It can also be used to test all types of equipment, not only that of one manufacturer in particular. Thus, it can be used to test CISCO, Juniper, SOPHOS, Citrix and many others. It is also multiplatform and can be installed on Windows, Linux and Mac operating systems. It uses a framework of open-source programs, each of which can emulate specific platforms to execute software programs corresponding to a provider. It is used to test configurations that will be deployed in the future on real equipment.

GNS3 is a network emulation tool similar to the Cisco tool PacketTracer, which is used to simulate a network infrastructure based on Cisco equipment and to configure equipment in order to reproduce the real infrastructure as closely as possible. GNS3 makes it possible to emulate networks more realistically, through both its ability to import real Cisco (or other) IOS into the routers emulated, and its ability to include in our virtual network real virtual hosts created with VirtualBox or other virtualization software. Thus, with GNS3, we can link the real to the virtual.

It includes the following components:

– Dynamips: emulates Cisco (or other) routers and switches and provides the corresponding environments or IOS;

– Dynagen: represents the Dynamips control and management software layer;

– Qemu: an emulator that enables GNS3 to execute Cisco ASA, PIX;

– VirtualBox or VMware: used to create and run virtual machines linked to Windows and Linux operating systems.

GNS3 is based on three concepts: simulation, emulation and virtualization. Since we have already discussed the concept of simulation, we will now introduce the other two concepts.

Emulation does not allow modeling, but actually identically reproduces the behavior of a software program and its hardware architecture. Virtualization uses the architecture of the host system, while emulation reproduces its software, which makes the virtualization perform more efficiently.