IPv4 and IPv6 are the addressing schemes used to identify the sender and receiver machine(computer or other device) over the internet. Each host requires an IP address to communicate over the net.
IPv4 is the current version of providing addresses (though IPv6 is there for quite some time now). But because of the growth of internet, there is a need to provide more IP addresses then available in IPv4. The total addresses available in IPv4 are exhausting fast (The last top level (/8) block of free IPv4 addresses was assigned in February 2011 by IANA). Hence comes the newer version, IPv6.
How many unique addresses can be given in IPv6 ?