Filius is a network simulator that lets you build and configure your own network. Lets you install web servers, DNS, DHCP etc. and inspect packets.

There are some great instruction videos on Youtube

FILIUS was initially developed by the University Siegen, Germany, to provide a tool for enhancing computer science lessons on networks. The main target group are students of secondary schools but with its wide range of applications it can be interesting for learners of any age. The software especially promotes explorative learning and is very helpful to teach students about the internet and its various applications.

The English language version of the Introduction guide is here.

The Download can be found here.