We are given an IPv4 datagram with the following characteristics.

  • The source IP address is and destination is

  • Its data content is an unfragmented TCP message of 64 bytes (TCP header + data)

  • It has Type of Service value of 25, Identification of value 264, Time to Live of 254 (all decimal values)

  • It is disallowed from being fragmented in transmission and contains no options

We are going to calculate the value of the checksum field, step by step. Each answer is a 16-bit binary value, e.g. 1100111101010011.

As a first step, what is the value of the first 16 bits of the IP header? The thing that I am confused about from this question is that how do we use the internet header length field to show that the data is 64 bytes long or 16 words(32 bit words) as the internet header length field is only 4 bits so it can only show up to 15 32 bit words?
Thanks in advance for any help

1 Answer 1


RFC 791, Internet Protocol, Section 3, shows the header format. The length field is 2 bytes (16 bits), not 4 bits. It explains that this allows for 65,535 octets.

  • I am referring too the IHL field which is 4 bits and represents the length of the internet header in 32 bit words. Do you know what it would be in this scenario Thanks
    – Abo abdu
    Oct 8, 2015 at 17:44
  • 2
    The IHL only represents the length oh the IP header, not the payload (data).
    – Ron Maupin
    Oct 8, 2015 at 17:46
  • Oh, that makes sense. So in this case since they didn't specify, do we just assign it with the minimum of 20bytes which is 5 words?
    – Abo abdu
    Oct 8, 2015 at 17:49
  • That's what it sounds like.
    – Ron Maupin
    Oct 8, 2015 at 17:52

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