Voice over IP technology has rapid growth on network convergence in deployment of converged networks within enterprises. Most enterprises today sit behind Firewalls and also use private IP addressing behind NATs (Network Address Translators). These NATs cause significant problems for multimedia over IP to work and function properly. The de-facto
Internet standard Internet Protocol (IP) is not secure. All types of business conducted through Internet, raised the requirement for a secure protocol. IP Security Working Group of IETF has been working on IPSec (IP Security) protocols to protect the IP traffic on the packet level. There are problems faced when an IPSec traffic is intended to pass through a NAT device. NAT and IPSec is very hard to use together without a standard way which would provide simple management and interoperability between vendors. Nowadays there are many techniques and solutions are used to allow popular applications to work through NATs. In this paper we present the details of the problem and issues associated with NATs and then survey some ways to solve this problem in SIP which aspire to become the dominant protocol on the internet for establishing calls. As a case study, we simulate a VoIP network and study the behavior and quality of VoIP under this model. We study all the potential parameters that can deteriorate the quality of VoIP such as jitter, End-To-End delay, packet delay variation, and the effect of adding security to VoIP networks that has been measured using OPNET (Network
Simulation Tool).