In IPv4 header, which statement correctly describes Total Length and IHL?

• A. Total Length indicates the entire IPv4 packet length in bytes (header plus data); IHL indicates the header length in 32-bit words.
• B. Total Length indicates the header length; IHL indicates the payload length.
• C. Total Length indicates the data length only; IHL indicates 16-bit words.
• D. Total Length indicates the packet version; IHL indicates header checksum.

Answer: (A)

The main idea is understanding what these two IPv4 header fields represent. The Total Length field is 16 bits that indicate the entire IPv4 packet length in bytes, which includes both the header and the data. The IHL, or Internet Header Length, is a 4-bit field that specifies the header length in 32-bit words; to get the header length in bytes, you multiply by 4. So the header can vary in size (minimum 20 bytes when IHL is 5, up to 60 bytes when IHL is 15), and the data payload length is the Total Length minus the header length (in bytes). This is why the statement describing Total Length as the full packet length and IHL as the header length in 32-bit words is correct. The other options mix up which portion is counted or what IHL measures (for example, confusing Total Length with just the header or data, or mis-stating IHL as 16-bit or as related to checksum).