GNS3
From charlesreid1
GNS3 is a virtual network appliance for simulating software-defined networks. This is a cheap way of setting up a big network testbed to prepare for certifications like the CCIE (Cisco Certified Internetwork Expert).
Download: https://gns3.com/software
GNS3 virtual machine: http://docs.gns3.com/1Bn-s1Izkjp13HxcPF4b8QSGfkWJYG_dpMt9U1DQjvZ4/
GNS3 installation on Mac: http://docs.gns3.com/1MlG-VjkfQVEDVwGMxE3sJ15eU2KTDsktnZZH8HSR-IQ/