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Fast development of the Internet in recent years has led to the exhaustion of IPv4 addresses. Despite exert of various techniques aimed at economical use of available pool, such as NAT, depletion of available addresses could not be avoided, but only moved in time. This situation has caused intensification of interests in IPv6 protocol, which increases available amount of addresses and simultaneously simplifies structure of header being added to a segment in encapsulation process. Moving billions of computers and other network devices from IPv4 to IPv6 is not possible immediately. This process will take many years and knowing that, network equipment manufacturers have developed tools that enable coexistence of two versions of IP, both on the Internet and in local area networks. One of the largest manufacturers of network devices is Cisco. Cisco in IOS operating system that is used by routers and switches, has implemented a number of important methods to ensure smooth functioning of both IPv4 and IPv6. These methods are: dual stack, point to point tunneling (manually configured tunnels and GRE tunnels), point to multipoint tunneling (6to4 tunnels or ISATAP tunnels) and NAT-PT mechanism.
Primary method for the coexistence of IPv6 and IPv4 in a network is use of dual stack. It consists in configure IPv6 routing protocols and IPv6 addresses on all routers and hosts in a network, that have to forward IPv6 packets. This is the method, that will be perfect for relatively small networks, where simultaneous enabling IPv6 support will not be a problem. Wherever this method, due to the number of routers, is impossible to implement, it's recommended to use tunneling- encapsulation of an IPv6 packet in an IPv4 packet.
Point-to-point tunnels are best used when IPv6 traffic is high and regular. The differences between MCT and GRE tunnels are minor. They consist in a different header attached in encapsulation process. GRE has an additional header, placed between the IPv4 and IPv6 headers, which has a field of protocol type. This field allows sending of additional passenger protocols.
Multipoint tunnels are better option when IPv6 traffic is occasional and its size is difficult to predict. They are better in this situation, because their use is associated with a dynamic transmission of each packet, which in turn uses a lot of router resources. In contrast to point-to-point tunnels, multipoint tunnels do not support IGP IPv6 protocol and require the use of static routes or BGP. In the multipoint topology, new devices can be connected to the tunnel without performing additional configuration on existing routers. The main differences between the two tested methods of multipoint tunneling, except for how to configure them, consists in the possibility of using reserved IPv6 address prefix for 6to4 and various IPv6 quartets storing IPv4 destination addresses (quartet 2 and 3 for 6to4, 7 and 8 for ISATAP).
Last examined method - NAT-PT, allows communication between two hosts, one of which supports only IPv6 and second - IPv4. This mechanism consists in translating IP addresses, IPv4 and IPv6 headers and others, such as TCP, UDP or ICMP.
Despite the variety of available methods of implementation the coexistence of IPv4 and IPv6 protocols, it is recommended to use dual stack wherever possible. The use of other methods should be considered only after ensuring that double-stack is impossible or too difficult to apply.