Probably the only free Networking software to design and build networks of any size without the need for hardware (Cisco charges almost 200 $ for Cisco VIRL). GNS3 is used by the world’s largest community of network professionals and learners. GNS3 install is easy and easy to use. It has good online support from community. Therte are wide variety of vendors who support their packages with GNS3, many of them come completely free (CISCO VIRL might catch up soon).
My first experience was with GNS3 and I remembered when I made my Tutorials and wanted to share it with Cisco Academy the Manager told me GNS3 is not standard! and now it’s famous and can use it even from inside the Cisco Academy. After that Cisco recognized how simulators is very important! but GNS3 still more wide than CISCO VIRL. All IT guys move towards open source software and if CISCO VIRL become open source will kill GNS3..I think!