User Mode Linux

If you wish you create test network configurations on a single linux box , uml can do a lot for you. Ive been running as many as 4 uml instances on my debian box without any problems.All you need to get started with is a root file system and a kernel.

Download the rootfs of your choice from the uml homepage and build the kernel with ARCH=um.

Apart from rootfs and kernel , you need to do some configurations on your system like creating virtual block devices(i.e ubd0) for the filesystem and creating a tap device for networking.

On debian you can install uml-utilities package which provides some uml specific programs which you might need later on to try different network configurations.