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.
  * Pemu, a Cisco PIX firewall emulator based on Qemu.

GNS3 is an excellent complementary tool to real labs for Cisco network
engineers, administrators and people wanting to pass certifications
such as CCNA, CCNP, CCIP or CCIE. It can also be used to experiment
features of Cisco IOS or to check configurations that need to be
deployed later on real routers.

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

Important notice: users have to provide their own Cisco IOS to use
with GNS3. It is also recommended to read the documentation _before_
starting the program. As of version 0.8.7 the package for dynamips
has to be installed separately.  Port 127.0.0.1:7200 has to be open on
your firewall.