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 through 9 were assigned to experimental protocols designed to replace IPv4: SIPP (known nowadays as IPv6), TP/IX, PIP, and TUBA. Of these, only IPv6 is still in use.