The legacy version of IP, IPv4 is the widely deployed foundation protocol on which the Internet is built. Developed in the 1970s and documented in IETF RFC 791 (1981), IPv4 is a Network Layer (Layer 3) connectionless protocol for the routing of datagrams through gateways connecting networks and subnetworks. IPv4 also can be characterized as a best effort protocol, as it offers no guarantees of delivery, no sequencing, and no error detection and correction mechanism. IPv4 provides for packet segmentation and reassembly and provides specific addressing conventions in the form of dotted decimal notation. IPv4 supports routing control, as well as status translation and communications. While IPv4 has no concept of the specific content of the packet or of its service requirements, it also supports multiple service types, including low-delay, high-bandwidth, and high-reliability paths. In the 30+ years since IPv4 was introduced, the Internet (and the world, in general) has become a much more complicated, more intense, more populated, and less secure place. In a contemporary context, IPv4 is criticized for its inadequate address space and lack of inherent security mechanisms. IPv6, the current version resolves those issues, but is incompatible with older routers and other legacy Internet infrastructure. So, IPv4 remains the dominant protocol of the Internet. Although IPv4 can be used on a standalone basis, it more typically is used in conjunction with higher layer protocols in the TCP/IP protocol suite. The total size of the IPv4 datagram, including the header (shown in Figure I-1), can be up to 65,535 octets in length.At a minimum, all networks must support a packet of at least 576 octets.As illustrated in Figure I-1, the minimum size of the IP header is 20 octets.The IPv4 datagram contains the following fields: