GNS3 is an excellent complementary tool to real labs for network engineers, administrators and people wanting to study for certifications such as Cisco CCNA, CCNP, CCIP and CCIE as well as Juniper JNCIA, JNCIS and 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.
Thanks to VirtualBox integration, now even system engineers and administrators can take advantage of GNS3 to make labs and study for Redhat (RHCE, RHCT), Microsoft (MSCE, MSCA), Novell (CLP) and many other vendor certifications.
This project is an open source, free program that may be used on multiple operating systems, including Windows, Linux, and MacOS X. Source
FROM THIS LINK YOU CAN DOWNLOAD GNS3