A 4-bit field in a datagram or IP packet header. It gives the datagram header length measured in 32-bit words. The size of the data area can be computed by subtracting the length of the header (HLEN) from the total length field, which is 16 bits long - and the maximum possible datagram size is 65,535 octets (so far). All header fields have fixed lengths with the exception of the Options field and the Padding field.