Which tools or softwares are mostly used for the purpose of modeling a computer network?
I suppose you mean simulating computer networks? because modelling is different (you can model your network as a graph for instance). If this is the case have a look at these simulators:
but be careful before starting working with any "network simulator" because in many cases you don't need complex simulators. A simple C++, Java or Python code would be sufficient. (Actually, it is always a good programming exercise to create your own discrete-event simulator for a network).
If you are interested in distributed processors simulation (from a theoretical point of view), then I would suggest erlang (a programming language). It is functional language very similar to Haskell (created by Eriksson). it is very easy to create processes with identities. But even more, you can let the processors exchange messages by sending/receiving messages in a very simple way.