Internet which plays an important role in the communication all across the world, has grown widely as a result of extensive efforts in sharing information and expansion of widespread applications. Cisco reported that the number of connected portable devices to the internet will exceed the world population in 2012. The latest report about global data communication via portable devices that has been released by Cisco predicts that more than 10 billion portable devices such as tablets and cell phones will be connected to the internet all over the world by 2016. Development of voice and video communications over the internet, advent of IP based mobile, expansion of wireless internet, increase in the number of users and in general has increased the demand for IP addresses.But,32 bit address space of current protocol of internet has constrained IPv4 ability in allocating addresses to users. Although some mechanisms such as NAT and CIDR may expand the IPv4 life for a while and delay the IPv4 adress exhaustion but address space limitations and some intrinsic problems of IPv4 header remain as before. Therefore transition to the new version of the IP protocol IPv6 for solving the limited address space and other problems of IPv4 protocol is inevitable. IPv6 stands for “Internet protocol version 6”. This protocol has been designed by IETF to solve many problems of current and previous versions of the protocol. IPv6, as a substitution for IPv4, has solved the problem of IPv4 address space by assigning 128 bits for internet addresses. Furthermore, it has added new features such as mobility, security and automatic configuration to IP-based networks. However, it should be noted that we cannot consider a specific day for migration of all networks to IPv6, since IPv4 networks and services have been spread widely all over the world. In addition, most of the organizations have such a deep dependency to the Internet, which they cannot tolerate the internet interrupt even for single day. So, it is necessary to consider a period of time for network migration from current IPv4 to IPv6 which is called the coexistence time. Three main mechanisms have been proposed in the literature for the transition from current networks to IPv6.These mechanisms are: tunneling IPv6 packets in IPv4 packets and vice versa, Dual Stack mechanism and address translation between these two protocols. To provide a smooth transition while maintaining the current equipment, upgrades of network should be done based on the current IPv4 network. IPv6 should be implemented in a smaller scale and then it should gradually extended to larger environments. In this thesis, we investigate IPv6 protocols, its features and its facing challenges. An experimental test bed based on Isfahan University of Technology network is implemented using OPNET software. Different coexistence mechanisms of the two protocols have been simulated and compared using this test bed in various conditions. An Alysis of the results and We also simulate different suggested scenarios to determine which mechanism in which conditions would be better. Finally, based on the simulation results and review of the advantages or disadvantages of each mechanism, a comparative analysis of the results is presented, Key Words:The Internet Protocol IPv4,IPv6,IPv4 and IPv6 Coexistence,Tunneling, Enapsulation ,Routers