What do you use?
There are a lot of different network simulators dependant on how detailed you want to do your sim, and what kind of network you want to simulate.
NEURON and GENESIS are good if you want to simulate full biological networks (Which I'm geussing you probably don't) even down to the behaviour of dendrites etc.
NEST and SPLIT and some others are good for doing population simulations where you create the population on a node-by-node basis and see what the whole population does. This is pretty much the 'industry' standard approach, and is used a lot in research and commercial applications, so there are worth looking into. I know that IBM use SPLIT for some of their research.
MIIND is good if you want to use differential equations to model what a population would do, but this approach is relatively new and computationally expensive (if very cool).
Not sure if that is exactly what you wanted!
(N.B. if you google any of the names in caps along with the word "simulator" you will end up at the relevant web page =)