The Options field is an optional, variable-length part of an IPv4 header that carries additional header information. It can be 0 to 40 bytes long and is rarely used. section 7.1.10
Its presence makes the IPv4 header variable in length: the header is normally 20 bytes, but can reach 60 bytes when the Options field is 40 bytes. section 7.1
Reading the IHL field
An IHL value of 5 indicates a 20-byte header and no Options field. Any value above 5 indicates that Options are present; the maximum IHL value, 15, indicates a 60-byte header. section 7.1.2
Figure 7.1 The format of the IPv4 header. The header is typically 20 bytes in size (the minimum size) but can be up to 60 bytes if the Options field is used.
Use the IPv4 header diagram to locate the Options field and compare the minimum and maximum header sizes. section 7.1
