Question

In an IPv4 datagram, the M bit is 0, the value of HLEN is 10, the value of total length is 400 and the fragment offset value is 300. The position of the datagram, the sequence numbers of the first and the last bytes of the payload, respectively are

(A) Last fragment, 2400 and 2789

(B) First fragment, 2400 and 2759

(C) Last fragment, 2400 and 2759

(D) Middle fragment, 300 and 689

Answer 1

(C) Last fragment, 2400 and 2759

M = 0 – Means there is no fragment after this, i.e. Last fragment 

HLEN = 10 - So header length is 4 x 10 = 40, as 4 is constant scale factor 

Total Length = 400(40 Byte Header + 360 Byte Payload) 

Fragment Offset = 300, that means 300 x 8 Byte = 2400 bytes are before this last fragment 

So the position of datagram is last fragment 

Sequence number of First Byte of Payload = 2400 (as 0 to 2399 Sequence no are used) 

Sequence number of Last Byte of Payload = 2400 + 360 - 1 = 2759