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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Overview, Changes and Transition

IPv6 Motivation and Overview 1 Transition from IPv4 to IPv6

In the preceding overview, I explained that the primary motivator for creating a new version of IP was to fix the problems with addressing under IPv4. But as we also saw there, numerous other design goals existed for the new protocol as well. Once the decision was made to take the significant step of creating a new version of a protocol as important as IP, it made sense to use the opportunity to make as many improvements as possible.

Of course, there is still the problem of the pain of change to worry about, so each potential change or addition in IPv6 had to have benefits that would outweigh its costs. The resulting design does a good job of providing useful advantages while maintaining most of the core of the original Internet Protocol. The following list provides a summary of the most important changes between IPv4 and IPv6, showing some of the ways that the IPv6 team met the design goals for the new protocol:

• Larger Address Space: IPv6 addresses are 128 bits long instead of 32 bits. This expands the address space from around 4 billion addresses to, well, an astronomic number (over 300 trillion trillion trillion addresses).
  
  
• Hierarchical Address Space: One reason why the IPv6 address size was expanded so much was to allow it to be hierarchically divided to provide a large number of each of many classes of addresses.
  
  
• Hierarchical Assignment of Unicast Addresses: A special global unicast address format was created to allow addresses to be easily allocated across the entire Internet. It allows for multiple levels of network and subnetwork hierarchies both at the ISP and organizational level. It also permits generating IP addresses based on underlying hardware interface device IDs such as Ethernet MAC addresses.
  
  
• Better Support for Non-Unicast Addressing: Support for multicasting is improved, and support is added for a new type of addressing: anycast addressing. This new kind of addressing basically says “deliver this message to the easiest-to-reach member of this group”, and potentially enables new types of messaging functionality.
  
  
• Autoconfiguration and Renumbering: A provision is included to allow easier autoconfiguration of hosts and renumbering of the IP addresses in networks and subnetworks as needed. A technique also exists for renumbering router addresses.
  
  
• New Datagram Format: The IP datagram format has been redefined and given new capabilities. The main header of each IP datagram has been streamlined, and support added for easily extending the header for datagrams requiring more control information.
  
  
• Support for Quality of Service: IPv6 datagrams include QoS features, allowing better support for multimedia and other applications requiring quality of service.
  
  
• Security Support: Security support is designed into IPv6 using the authentication and encryption extension headers and other features.
  
  
• Updated Fragmentation and Reassembly Procedures: The way that fragmentation and reassembly of datagrams works has been changed in IPv6, to improve efficiency of routing and better reflect the realities of today's networks.
  
  
• Modernized Routing Support: The IPv6 protocol is designed to support modern routing systems, and to allow expansion as the Internet grows.
  
  
• Transition Capabilities: Since it was recognized from the start that going from IPv4 to IPv6 is a big move, support for the IPv4/IPv6 transition has been provided in numerous areas. This includes a plan for interoperating IPv4 and IPv6 networks, mapping between IPv4 and IPv6 addresses and more.

The rest of the sections on IPv6 provide much more detail on these changes and additions to IP. You'll notice that the majority of these are related to addressing, since that is where the greatest number of important changes have been made in IPv6. Of course, routing and addressing are closely related, and the changes to addressing has had a big impact on routing as well.

Another change that I should mention is that with the introduction of IPv6, several other TCP/IP protocols that deal intimately with IP have also had to be updated. One of these is ICMP, the most important support protocol for IPv4, which has been revised through the creation of ICMPv6 for IPv6. An addition to TCP/IP is the Neighbor Discovery (ND) protocol, which performs several functions for IPv6 that were done by ARP and ICMP in version 4.

IPv6 Motivation and Overview 1 Transition from IPv4 to IPv6