Computer networks simulation is considered as one of the most powerful steps preceding any network implementation. This is because of the prior knowledge gained about performance and other network activities.The work in this thesis deals with simulation of different traffic generated networks, analyze them and figure out important parameters affecting their performance. Quality of service techniques are also included in the simulation. Results obtained show the change of traffic response time, delay, throughput and utilization when networks are subjected to certain constrains (ISP link for example). Three cases are designed, analyzed and simulated in this work. The first case shows the effect of ISP link bandwidth in bits/sec on corporate network connected to internet. The second case compares the traffic generated by three types of service sources with background traffic. Here weighted fair queuing QoS technique is used. The third case compares between QoS techniques implemented on routers firewalls. The results obtained clearly show the effect of these techniques on traffic queued in Bottleneck links. Overall results obtained with OPNET give a conclusion that most network parameters can be monitored and analyzed to reach an optimized network design.