Introduction
Before discussing on IPv6 (Internet Protocol version 6), let’s take a look on the history of IPv4 (Internet Protocol version 4). The Department of Defense Advanced Research Projects (DARPA or ARPA) had introduced the TCP/IP protocol.
DARPA organized research to communicate between remote computers in 1963. Researchers placed three terminals at main office of DARPA ; each terminal connected to a different remote computer. The remote computers were placed at:
SDC(System Development Corp.), CA
University of California – Berkeley
Multics at MIT
Researchers observed that it was not efficient to communicate with each remote computer on a different terminal. They realized that for efficient communications an intermediary device will be required. For that mini computers known as IMPs (Interface Message Processors) or routers were developed by mid – 1968.
Arpanet had installed four IMPs at UCLA, Stanford Research Institute, UC Santa Barbara, and the University of Utah. The first communication was established between the IMP at SRI and IMP of UCLA on 21st November 1969.The number of IMPs had grown from 4 to 40 by September 1973. By 1981, the number of hosts had increased from 4 to 213.
The developers of ARPAnet had not imagined the growth what is now known as Internet. It was not imagined by any one that the IPv4 address structure would fall short of the addresses required. Even no one could foresee all the devices that would require IP addressing. Now the IPV4 addresses are going to be exhausted.

The Requirement of Larger Address Space:
Emerging Internet population
Mobile users – PDA, Pen Tablet etc.
Cellular Phones
Transportation – Internet access in trains and planes
Electronic devices

With implementing IPv6, the address space is upgrading from 32-bit address to a 128-bit address. Every person on the earth can be allowed more than 1000 addresses by this new address structure. Some countries as China and Japan have adopted IPv6.