Which RFC defines the IPv4 protocol?

RFC 791 is the Internet Engineering Task Force (IETF) document that formally defines the Internet Protocol version 4 (IPv4). Published in September 1981, RFC 791 outlines the specifications concerning IP addressing, packet structure, and the fundamental concepts of IPv4, including delineation of datagrams, options, and fragmentation, among other features critical to the operation of IP networks. This document serves as a cornerstone for understanding how IPv4 operates and continues to be a foundational reference in the networking community.

In contrast, RFC 5246 relates to Transport Layer Security (TLS), RFC 3439 discusses the architectural principles of the Internet, and RFC 2460 defines Internet Protocol version 6 (IPv6), all of which are important in their own right but do not pertain specifically to IPv4.