The current version of Internet Protocol (IP) in wide deployment is version 4.  Version 6 is currently in use on a number of experimental networks and will ultimately replace IPv4 on the internet.  There is however, a lot of work yet to be done before this can happen.  It is important to understand that although the most IT exams are concerned with subnetting in IPv4, you should be familiar with how to read IPv6 (see later) because at some point in the not too distant future it will soon become the standard for all networks.