IPv4 is the fourth version of the communication protocol underlying the Internet and the first version to be widely deployed.
First formalized in 1981, IPv4 traces its roots to the initial development work for ARPAnet. IPv4 is a connectionless protocol to be used on packet-switched Link layer networks (ethernet). IPv6 is gradually replacing IPv4 owing to IPv4's security problems and the limitations of its address field. (Version number 5 was assigned in 1979 to the experimental Internet Stream Protocol, which however has never been called IPv5.)
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IPv4 on Wikipedia