Madeeasy Test series CN Q2

Question

Which of the following is/are correct regarding IPV4 Datagram Header?

1. The minimum length of HLEN field is 5 and maximum is 20.
2. Total length field has a minimum value of 20 Bytes and maximum of 65516 Bytes
3. Fragmentation offset maximum value is 65514 Bytes.
4. None of the above

Comments

only c)

---

How is C possible ?

---

my bad, i think none is correct so option d)

---

No actually they gave c. But how is c possible ? The Fragmentation offset size is of 13 bits only which can have a maximum value of 8191 i.e., 8191 * 8 = 65528 Bytes.

---

https://www.geeksforgeeks.org/gate-sudo-gate-2020-mock-iii-24-january-2019-question-20/

---

https://en.wikipedia.org/wiki/IP_fragmentation_attack#:~:text=An%20IP%20header%20is%20at,from%20another%20at%20the%20destination.

But Wiki says, maximum offset value is 8189 i.e., 8189 * 8 = 65512 B

Answer 1

1. Header length (20 B to 60 B) so HLEN can go from 5 to 15 .
2. Total length (16-bit) means total length of  packet ( including Header and Data). The minimum size is 20 bytes (header without data) and the maximum is 65,535 bytes.
3. Fragmentation offset 13 bit : Maximum fragment offset possible = (65535 – 20) – 1 = 65514 where 65535 is maximum size of datagram and 20 is minimum size of IP header. so Option C is true.

Comments

why  -1 subtracted ?