VIRL is a network simulation platform that allows network operating systems, to run in a virtual environment. It provides a drag-and-drop user interface to design network topologies, an automated configuration engine to configure IP addressing and routing protocols, and a simulation engine based on OpenStack, allowing clustering across multiple physical servers. The VIRL platform includes virtualised versions of Cisco's four main routing operating systems (IOS, IOS-XR, IOS-XE and NX-OS), which can be connected to other network devices such as firewalls, layer 2 switches, Linux servers, or connected to external devices such as live BGP feeds or physical hardware. With the combination of real-world operating systems (using the same control-plane code as production routers), traffic generators and automated packet capture, VIRL provides a comprehensive platform for conducting network research, change validation, training, education, and network applications development. This talk will cover the VIRL platform architecture, focusing on the key components as well as looking at how such Network simulation system are being used in the real world.