IPv4 header length field

The header of an IPv4 packet contains what is known as the Header Length field.

This is a 4-bit field that tells us the length of the IP header in 32-bit increments, or in 4-byte increments. The minimum length of an IP header is 20 bytes so with 4-byte increments, the minimum value one would see is 5. The maximum value we can create with 4 bits is 15 so with 4-byte increments, that would be a header length of 60 bytes. This field is also called the Internet Header Length (IHL).

Links

https://networklessons.com/cisco/ccnp-encor/ipv4-packet-header