GNS3 is a excellent complementary tool to real labs for
administrators of Cisco networks or people wanting to pass their CCNA,
CCNP, CCIP or CCIE certifications.
It can also be
used to experiment features of Cisco IOS or to check configurations
that need to be deployed later on real routers. This project is an open
source product that may be used on multiple platforms, including
Windows, Linux, and MacOS X.
Features overview
Designing high quality complex network topologies.
Emulating Cisco routers and PIX firewalls.
Simulating simple Ethernet, ATM and Frame Relay switches.
Load and save in Dynagen's INI-like format.
Image export (JPEG, PNG, BMP and XPM).
Important notice: users must provide their own Cisco IOS to use GNS3. http://www.gns3.net/