Looking for network simulator (subnets, routers, etc..)

You could take a look at these:

GNS3

visual - netkit

OMNeT++

I've use d GNS3 in the past and it was pretty good, I haven't any personal experience with the other two.

NS2 is the open source standard, while Opnet is the commercial standard. NS2 is gradually being replaced by NS3.

I've used NS2 extensively (while working at Qualcomm) and it's very powerful. See also the entry on network simulation on wikipedia, which also lists these applications.

from my own experience: 1- Packet tracer: is just for learning. they taught us when we were taking CCNA, and cisco materials. It is quite simple and GUI oriented.Normally, it a good idea to show that your know it and tell that it is simple.and it could serve the purpose of subnetting, routing, wireless, and so many things , but as I said , it is quite simple.

however, when you are talking about network simulators through which you can do your own modification, your can not use packet tracer, NS2, Omnet++,and NS3 should be your options. These simulators are so powerful and need some time to get it. My recommendation for a starter, Omnet++ has some good GUI basic modules, it allows you to learn simple things easily,refer to some basic tutorials and you could get started with your project within short time. NS2 or NS3 they are quite hard(in my opinion) and they do not tend to have GUI. To make it easy for you, i think you should go for omnet++ and here is the basic tutorial .