Twenty years ago, there were many competing networking protocols. Today, one protocol is overwhelmingly common—the Internet Protocol. It comes in two versions—IPv4 and IPv6. IPv4 is completely ubiquitous and deployed everywhere. If you're deploying network code today, you must support IPv4 or risk that a significant portion of your users won't be able to connect.
IPv4 uses 32-bit addresses, which limits it to addressing no more than 232 or 4,294,967,296 systems. However, these 4.3 billion addresses were not initially assigned efficiently, and now many Internet Service Providers (ISPs) are forced to ration IPv4 addresses.
IPv6 was designed to replace IPv4 and has been standardized by the Internet Engineering Task Force (IETF) since 1998. It uses a 128-bit address, which allows it to address a theoretical 2128 = 340,282,366,920,938,463,463...
