It's in the spec. From RFC 791, "Internet Protocol version 4", page 13 (https://tools.ietf.org/html/rfc791 https://tools.ietf.org/html/rfc791): Total Leng

It's in the spec. From RFC 791, "Internet Protocol version 4", page 13 (https://tools.ietf.org/html/rfc791 https://tools.ietf.org/html/rfc791): Total Length is the length of the datagram, measured in octets, including internet header and data. This field allows the length of a datagram to be up to 65,535 octets. Such long datagrams are impractical for most hosts and networks. All hosts must be prepared to accept datagrams of up to 576 octets (whether they arrive whole or in fragments). It is recommended that hosts only send datagrams larger than 576 octets if they have assurance that the destination is prepared to accept the larger datagrams. The number 576 is selected to allow a reasonable sized data block to be transmitted in addition to the required header information. For example, this size allows a data block of 512 octets plus 64 header octets to fit in a datagram. The maximal internet header is 60 octets, and a typical internet header is 20 octets, allowing a margin for headers of higher level protocols. Note: That was published in 1981, when internetwork speeds were likely around 1 Mbps or lower.