In the network research area the implementation of any network in real world is not easily possible because of establishing the network in real world is time consuming and very much costly. The simulator helps the network developer to check whether network works properly or not in the real world. Network simulator is used to test new networking protocols or change the existing protocols in a controlled and reproducible environment. One can design networks by using various types of nodes, hubs, bridges, Routers and switches with the help of network simulator. It enables the user to specify the node on the network, the link between those nodes and the traffic between the nodes. Typically users can customize the network simulator to fulfill their specific analysis needs. In the current scenario various network simulators are available like OPNET, QualNet etc. but all these software are commercial that means one have to pay to get license to use their software. Therefore we focus on open source simulator (NS2 and NS3) that is open for everyone. The advantages of open source simulator is that it is very flexible and reflect the most new recent development of new technologies. ukryj opis