Most of today's Internet uses IPv4, which is now nearly twenty years old.  IPv4 has been remarkably resilient in spite of its age, but it is beginning to have problems. Most importantly, there is a growing shortage of IPv4 addresses, which are needed by all new machines added to the Internet.

IPv6 fixes a number of problems in IPv4, such as the limited number of available IPv4 addresses. It also adds many improvements to IPv4 in areas such as routing and network autoconfiguration. IPv6 is expected to gradually replace IPv4, with the two coexisting for a number of years during a transition period.

The IP addresses that are currently assigned to computers are called IPv4 addresses, and they are 32-bits in length. IPv4 address is written as 210.130.1.1, for example. IPv6 addresses are the 128-bit successors to IPv4 addresses, and written as 2001:0db8:3c4d:0015:0000:0000:1a2f:1a2b and can be abbreviated as 2001:db8:3c4d:15::1a2f:1a2b.  IPv6 defines three address types: unicast - identifies an interface of an individual node; multicast - identifies a group of interfaces, usually on different nodes. Packets that are sent to the multicast address go to all members of the multicast group; and anycast - identifies a group of interfaces, usually on different nodes. Packets that are sent to the anycast address go to the anycast group member node that is physically closest to the sender.

IPv6 is designed to run well on high performance networks (e.g. Gigabit Ethernet, OC-12, ATM, etc.) and at the same time still be efficient for low bandwidth networks (e.g. wireless). In addition, it provides a platform for new internet functionality that will be required in the near future. It also includes a transition mechanism which is designed to allow users to adopt and deploy IPv6 in a highly diffuse fashion and to provide direct interoperability between IPv4 and IPv6 hosts. The transition to a new version of the Internet Protocol must be incremental, with few or no critical interdependencies, if it is to succeed. The IPv6 transition allows the users to upgrade their hosts to IPv6, and the network operators to deploy IPv6 in routers, with very little coordination between the two.

The introduction of IPv6 will not merely increase in the number of IP addresses, rather, it will herald a shift in our paradigm about what can be done on the Internet.  IPv6 aims to provide intelligent transmission framework that is easy to handle for intermediate devices by keeping the basic header simple and fixed length. 

In case you're wondering why IPv6 was used as the successor of IPv4 and not IPv5, on any IP header, the first 4 bits are reserved for protocol version. So theoretically a protocol number between 0 and 15 is possible.  4 is already used for IPv4, 5 is reserved for the Stream Protocol, the next free number was 6. Hence IPv6 was born.

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