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
The best network simulator; you can integrate anything into it.