An advanced network simulator to design and configure virtual networks
Build, Design and Test your network in a risk-free virtual environment and access the largest networking community to help. Whether you are studying for your first networking exam or building out a state-wide telecommunications network, GNS3 offers an easy way to design and build networks of any size without the need for hardware.
The Common Open Research Emulator (CORE) is a tool for emulating networks on one or more machines. You can connect these emulated networks to live networks. CORE consists of a GUI for drawing topologies of lightweight virtual machines, and Python modules for scripting network emulation.
niab - Network In A Box.
Create a virtual lab network inside one machine. A lab can include routers, firewalls, clients and servers connected by a network specified by you. [Linux Network Simulator, UML, user mode linux]