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A host X send a message to host Y having two intermediate  networks in between them.A TCP message consisting of 2100 bytes is passed to IP for delivery across two hosts. In first network has MTU of  1200 bytes  second network has MTU of 400 bytes excluding network overhead. Assume that IP overhead per packet is 20 bytes. What is the fragmentation offset for the Last fragment that reaches destination

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File size = 2100 when it goes to network layer 20 bytes are added so it becomes 2120B but this is just for understanding as we can solve it without considering IP overhead because fragmentation is done on data not headers.

At sender S : 2100 B to intermediate router P(1200 bytes without header) to intermediate router Q(400 B w/o header)  to destination D.

S to P :

fragement 1 : 1200 fragment 2 : 900 to P

P to Q :

for 1200B fragment 

F1 : 400(data length in bytes) offset : 0 

F2 : 400 offset : 400/8=50

F3 : 400 offset : 100

for 900B fragment 

F4: 400 offset : 150

F5: 400 offset : 200

F6: 100 offset : 250 last fragment 

In other way 250*8 = 2000 bytes ahead of this last fragment (from 0 to 1999) and sequence number of last fragment is 2000.

So 250 shoud be answer.

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