GNS3 is a graphical network simulator that allows simulation of complex networks.

To allow complete simulations, GNS3 is strongly linked with :

Dynamips, the core program that allows Cisco IOS emulation.
Dynagen, a text-based front-end for Dynamips.
Qemu, a generic and open source machine emulator and virtualizer.
GNS3 is an excellent complementary tool to real labs for network engineers, administrators and people wanting to pass certifications such as CCNA, CCNP, CCIP, CCIE, JNCIA, JNCIS, JNCIE.

It can also be used to experiment features of Cisco IOS, Juniper JunOS or to check configurations that need to be deployed later on real routers.

This project is an open source, free program that may be used on multiple operating systems, including Windows, Linux, and MacOS X.

Features overview

Design of high quality and complex network topologies.
Emulation of many Cisco IOS router platforms, IPS, PIX and ASA firewalls, JunOS.
Simulation of simple Ethernet, ATM and Frame Relay switches.
Connection of the simulated network to the real world!
Packet capture using Wireshark.