IPv4 Protocol – What is the Maximum Datagram Size for IPv4?

The Internet Protocol (IP) is defined in RFC 791. The RFC specifies the format of the IP header. In the header there is the IHL (Internet Header Length) field which is 4 bits long and specifies the header length in 32 bit words. The IHL field can hold values from 0 (Binary 0000) to 15 (Binary 1111).

So the longest Internet Header (IP header) size can be 15*32 Bits = 480 Bits = 60 Bytes. This is why the header has a maximum size of 60 Bytes.

The shortest header size is 20 bytes, where the IHL field has the value 5 (0101). This is because all the required fields in the header need 20 Bytes of space. So while in theory you could set the IHL to a value < 5 this would always be an incorrect value and thus an invalid packet header.

The IP fragmentation and reassembly is described by the RFCs. You must fragment on 64-bit boundaries. There are RFCs dealing with this, and other sites which will describe the fragmentation and reassembly process in depth; you can do a search to find them

Start with RFC 791, INTERNET PROTOCOL:

To fragment a long internet datagram, an internet protocol module
(for example, in a gateway), creates two new internet datagrams and
copies the contents of the internet header fields from the long
datagram into both new internet headers. The data of the long
datagram is divided into two portions on a 8 octet (64 bit) boundary
(the second portion might not be an integral multiple of 8 octets,
but the first must be). Call the number of 8 octet blocks in the
first portion NFB (for Number of Fragment Blocks). The first
portion of the data is placed in the first new internet datagram,
and the total length field is set to the length of the first