Nowadays, the Internet Protocol is facing the version change; IPv4 (the old version of IP) will be replaced by IPv6 (the new version of IP) in the long run. Although they are based on the very same concept, the two protocols are not compatible with each other. This chapter deals with the coexistence issues, which might arise due to the simultaneous existence of IPv4 and IPv6. On the other hand, this chapter also covers transition concepts: how IPv6 only hosts can communicate either over IPv4 only networks, or with IPv4 only hosts in the Internet of the future.
TopBackground
IPv4 is a standard (RFC791) from 1980. It was developed in an era when computers occupied huge rooms and networking was available at speeds of 300 bits/s. Since that time, many improvements have appeared, accepted, and employed, according to the needs of computer networks and communication elements (computers, routers, etc.) of the last few decades. These improvements are basically extensions (more RFCs) to the existing IPv4 standard. These RFCs describe the additional functionality (like multicasting) and the related protocols, which support the functionality. At last, in the beginning of 1990's the emerging IPv4 protocol suite resulted in an advanced communication protocol, which supports all the needs of Internet networks and nodes of that time.
However, it was visible that there is one property of the IPv4 protocol, which needs revision, but cannot be extended easily, since it is fixed deep in the roots of the protocol. It was addressing. An IPv4 address comprises four bytes (32 bits). Assuming uniqueness (that one IP address must be assigned to exactly one host in the whole world), this address space makes it possible to address a bit more than four billion hosts around the world. Four billion does not sound too much, as the number of people living on Earth exceeds 6.82 billion as of 10th March 2010. In the long run we can be sure that everybody will need Internet connection, and it is expected that more than one device will be connected to the Internet. Thus, in the long run, IPv4 will not be able to serve the whole humankind for sure.
Changing the addressing methodology in IPv4 would require to change not only all the RFCs which describe, or refer to addressing, but also all the hosts and network entities which are currently connected to the Internet. Clearly, this is not possible. Thus, Internet developers decided to release a new version of IP, namely IPv6.
IPv6 provides a huge address space, with addresses consisting of 128 bits. This address space could cover the surface of the Earth with an average density of 665 billion addresses/μm2. That is, in every square micrometer, there could fit 150 pieces of the whole IPv4 Internet. This amount of addresses is expected to be enough for the long run, even if humankind starts to populate the space. Addressing is the main motivator for changing the version, and also the most important property of the new protocol.
Now let us see what other novelties can be found in IPv6. Basically, IPv6 is a conceptual copy of the IPv4 protocol, with all the functionalities existing in IPv4. That is, if a professional is familiar with IPv4, she really does not need big efforts to learn IPv6. The real difference is basically the integrity of the standard. At the creation of IPv6, the authors were aware of the existing functionalities of IPv4: they tried to integrate all functionalities of IPv4 extensions into the basic IPv6 standards. Thus, the IPv6 standards are more complete, and thus IPv6 implementations have all the functionalities that are available in advanced IPv4 implementations.
Of course, there are new concepts, which can be found in IPv6 only. However, it must be noted that they are the minority. As long as IPv4 is the main protocol of the Internet (there are more hosts use IPv4 today than the ones that use IPv6), all new functions of IPv6 will be backported to IPv4 for sure.