Graphical Network Simulator-3 is a software emulator for networks, launched in 2008. It allows the combination of virtual devices and real devices, and so can be used to simulate complex networks. It uses Dynamips emulation software to simulate Cisco IOS.GNS3 is used by major corporations including Exxon, Walmart, AT&T and NASA, and is also popular with people preparing for network professional certification exams. As of 2015, the software had been downloaded 11 million times.