Link.

IPv4 is the current version of the Internet Protocol, the identification system the Internet uses to send information between devices. This system assigns a series of four numbers (each ranging from 0 to 255) to each device.

IPv4 only allows for about 4 billion addresses and the Internet needs more room than that. IPv6 is the new version of the Internet Protocol and expands the number of available addresses to a virtually limitless amount–340 trillion trillion trillion addresses.