RFC 791 defines IPv4 as the Internet Protocol version 4 specification.
The Internet Engineering Task Force published RFC 791 in September 1981. It describes the packet format and addressing rules for IPv4, including fields such as the source address, destination address, time to live, and protocol identifier. IPv4 uses 32-bit addresses, written commonly as four decimal octets.
IPv4 helped provide the addressing foundation for the expanding ARPANET and later the public Internet. Its address space contains about 4.3 billion possible addresses, although some are reserved and allocation is not perfectly uniform. The growth of connected devices eventually made IPv4 address exhaustion a major engineering problem.
Network address translation, private addressing, and address-conservation policies extended IPv4's practical life. IPv6 was designed as its successor and uses 128-bit addresses, but both protocols continue to operate in parallel across much of the Internet.