IPv4 stands for Internet Protocol version 4. It is the original standard set up for handling IP addresses when the Internet was initial developed by DARPA (Defense Advanced Research Projects Agency) in the early 1970s.
IPv4 uses a 32 bit address field which provides for 4,294,967,296 unique Internet addresses. This is the number of computers/devices that can be connected to and use the Internet. In the early 1970's the population of the earth was less than 4 billion people, personal computers did not exist, and at most there were hundreds, perhaps thousands of mainframe and mini computers that had been assigned Internet addresses. So the 4 billion plus address space was deemed to be more than enough to last beyond any foreseeable requirements.
IPv4 addresses are all but consumed
By 1992, the rapid explosion of the Internet fueled by the vast number of personal computers attaching to it, made it clear that the IPv4 address space was already consumed to the point that a replacement had to be found.
IPv6 was developed in response to this situation. IPv6 allocates 128 bits to map the Internet address space. The number of bits were not just doubled, but instead quadrupled from IPv4's 32 bits to insure that this address space would not run out any time soon!
IPv6 addresses will probably never run out
128 address bits provide IPv6 with 340,282,366,920,938,463,463,374,607,431,768,211,456 unique addresses. It may seem like overkill to have this many addresses available, However, many visionary individuals believe that eventually every wired and wireless computer, cell phone, PDA, household appliance, security camera, devices that haven't yet been invented, will each have their own unique Internet address.
Besides the huge number of IP addresses, IPv6 provides for better handling of voice than IPv4 which was not initially set up to handle it. This means that phone conversations over the Internet will be smooth and clear instead of choppy and broken up like they often are now.
The time is almost upon us when any device with an Internet address and a connection to the Internet can be monitored and controlled from anywhere in the world. While you're away on vacation you could turn on lights, change your thermostat, check security cameras around your home, etc. The possibilities are only limited by our imagination!