• Internet Protocol (IP) is the transmission mechanism used by TCP/IP, which is the suite of communication protocols used to connect hosts on the Internet.
• The purposes of IP: it defines the basic unit of data transfer used throughout TCP/IP (i.e. the format of data as it passes across the internet); the IP software performs the routing function (i.e. choosing a path over which data will be sent); and includes a set of rules that embody the idea of unreliable, connectionless and best-effort packet delivery.
• IP Addresses give a unique identifier to nodes which allow the transmission of data over the internet between two or more nodes.

IP provides an unreliable service (i.e., best effort delivery). This means that the network makes no guarantees about the packet and none, some, or all of the following may apply:

• data corruption

• out of order (packet A may be sent before packet B, but B can arrive before A)

• duplicate arrival

• lost or dropped/discarded

Through 9 were assigned to experimental protocols designed to replace IPv4: SIPP (known nowaday IP is the common element found in today’s public Internet. The current and most popular network layer protocol in use today is IPv4; this version of the protocol is assigned version 4. IPv4 RFC-791 was adopted by the United States Department of Defense as MIL-STD-1777.

IPv6 is the proposed successor to IPv4 whose most prominent change is the addressing. IPv4 uses 32-bit addresses (~4 billion addresses) while IPv6 uses 128-bit addresses (~3.4×1038 addresses). Although adoption of IPv6 has been slow, as of 2008, all United States government systems must support IPv6 (if only at the backbone level). [3]

Version numbers 0 through 3 were development versions of IPv4 used between 1977 and 1979. Version number 5 was used by the Internet Stream Protocol (IST), an experimental stream protocol. Version numbers 6 s as IPv6), TP/IX, PIP, and TUBA. Of these, only IPv6 is still in use.

• Wikipedia page on Internet Protoco
• Webopedia defintion