What is the minimum length of the IPv4 header?

The minimum length of the IPv4 header is indeed 20 bytes. This length is established by the standard format of the IPv4 header, which is defined in RFC 791. The header consists of a number of fields that provide crucial information for routing and delivering packets across networks.

The header starts with a 4-bit version field that specifies the IP version, followed by a 4-bit header length field, which indicates the size of the header in 32-bit words. The minimum value of this field is 5, which means 5 words or 20 bytes. Following this, the header continues with other fields such as the Type of Service, Total Length, Identification, Flags, Fragment Offset, Time to Live, Protocol, Header Checksum, Source Address, and Destination Address.

While it is conceivable to have longer headers (up to 60 bytes), as defined by the options for options in the header, a header length of 20 bytes is the baseline length required for any basic IPv4 packet to function properly.