IPv4 is the most widely used version of the Internet Protocol. It defines IP addresses in a 32-bit format, which looks like 126.96.36.199. Each three-digit section can include a number from 0 to 255, which means the total number of IPv4 addresses available is 4,294,967,296 (256 x 256 x 256 x 256 or 2^32).
Each computer or device connected to the Internet must have a unique IP address in order to communicate with other systems on the Internet. Because the number of systems connected to the Internet is quickly approaching the number of available IP addresses, IPv4 addresses are predicted to run out soon. When you consider that there are over 6 billion people in the world and many people have more than one system connected to the Internet (for example, at home, school, work, etc.), it is not surprising that roughly 4.3 billion addresses is not enough.
To solve this problem, a new 128-bit IP system, called IPv6, has been developed and is in the process of replacing the current IPv4 system. During this transitional process from IPv4 to IPv6, systems connected to the Internet may be assigned both an IPv4 and IPv6 address.