In an IPv4 header, what does the Internet Header Length (IHL) field specify?

• A. The Total Length field indicates the header length.
• B. The Time to Live field indicates the header length.
• C. The Internet Header Length (IHL) field indicates the header length in 32-bit words.
• D. The Protocol field indicates the header length.

Answer: (C)

The main idea here is that the Internet Header Length (IHL) tells how long the IPv4 header is. It’s a 4-bit field, and each unit equals one 32-bit word (4 bytes). So the header length in bytes is IHL multiplied by 4. The minimum value is 5, which means a 20-byte header with no options. If there are options, the IHL increases up to a maximum of 15, giving up to 60 bytes for the header. This tells the receiver where the header ends and the payload begins. By comparison, the Total Length field specifies the entire packet length (header plus data), the Time to Live field is about hop count, and the Protocol field indicates the next protocol after the header, not the header size.