ISRO CSE 2014 | Question: 31

Question

A IP packet has arrived in which the fragmentation offset value is $100,$ the value of $\textsf{HLEN}$ is $5$ and the value of total length field is $200.$ What is the number of the last byte?

• A. $194$
• B. $394$
• C. $979$
• D. $1179$

Answer 1

Data length $ = 200 – 4 \times 5=180$

Starting number of the first byte of fragment $ = 100\times8=800$

Number of the last byte $ = 800+179 = 979$

$\mathbf{Ans}\;\textbf C$

Comments

@ayush palak @Raju Kalagoni    But shouldn't data be a multiple of 8? Here data size is 180, which is not a multiple of 8.

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@shraddha priya, data need not be a multiple of 8. it doesn't make sense to have data always in multiple of 8. We should always be in a position to send data across network either in multiple of 8 or not.

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@shraddha priya Data should be a multiple of 8 in cases where there are more than 1 fragmented packets and all the packets except the last packet must be a multiple of 8. If there is only one packet or the packet is last fragmented packet then it need not be a multiple of 8. 

Answer 2

$\mathbf{Option \;C}$

Total Length=$200$

Data field excluding header=$ 200-20 = 180(0 \dots 179)$

No of byte ahead from this packet is $ = 100\times8 = 800$

so last byte = $800+179=979$

Answer 3

Header

Payload

An IP packet looks like this.

Header Length = $5*4=20$

So, Payload = $200-20=180$

 

Fragmentation offset = $100*8=800$

The last byte would be $180 + 800$th = $980th$ but since we number everything from 0 in computer science, it'll be $979th$

Answer 4

Here in this case answer should be 979.

Answer 5

Here is easy and simple and logical explanation