IP header length (IHL)
The IP Header Length (IHL) data field is in the header of various network protocols and transport protocols.
In the IP header it is a four-bit long data field in which the number of 32-bit words in the IP header is specified. Based on this data field, the data part following the IP header is calculated. This calculation is necessary because the length can change due to the option field, but it is always a multiple of 32 bits. This makes the maximum header length in the normal case, when no option is present, 15 times 4 bytes, i.e. 60 bytes. In this case, a 5 is entered in the data field for the header length. Since the fixed IP header has a length of 20 bytes, 40 bytes remain for the option field.
For the User Datagram Protocol( UDP), the header length field is 16 bits long and defines the length of the UDP header and the UDP data. The IP packet can be identified from the hexadecimal values of the version field and the IHL field. For a normal IP packet, these two data fields contain a 45.