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.)