Which RFC specifies Internet Protocol version 4?

RFC 791 specifies Internet Protocol version 4, commonly called IPv4. Published in September 1981, it defined the core protocol for delivering datagrams across interconnected packet-switched networks.

The document was edited by Jon Postel for the DARPA Internet Program and prepared at the University of Southern California’s Information Sciences Institute. It replaced RFC 760 and described how hosts and gateways should handle addressing, routing, fragmentation, reassembly, and the Internet datagram format.

IPv4 uses 32-bit addresses, traditionally written as four decimal octets such as 192.0.2.1. Its header includes fields such as version, total length, identification, time to live, protocol, header checksum, and source and destination addresses. IP itself does not guarantee delivery, ordering, or retransmission; those responsibilities belong to higher-level protocols such as TCP.

RFC 791 remains historically central, although later RFCs updated parts of IPv4, including the type-of-service field and identification-field behavior. IPv6 is specified separately and uses a different 128-bit addressing system.