The Internet Header Length (IHL) is used only in IPv4. The IHL counts the number of 32-bit words that make up the header. The minimum for this is 5 (no options or padding used). The maximum header length is 15 x 32 bits = 60 bytes since the IHL field can only hold 4 bits, which is a maximum value of 1111 (binary 15).

Options is a rarely used field, and has a number of different purposes. These include methods for dealing with fragmentation, the methods to be used during source routing, as well as a number of experimental features.

The different values for Options may not fill a 32-bit word, so padding bits are added in order to fill the word up. This allows the IHL field to indicate the start of the payload data.

In IPv6, the header length is fixed at 40 bytes (as in The IPv6 header diagram).