Question

Computer Networks
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Consider the hexadecimal numbers below which represents the first 20 bytes of an IP packet:

17 00 00 50 00 01 60 00 03 59 D1 A8 D8 3A C6 4E AC D9 12 E5 …..Based on packet, answer the following:

Answer:

Questions	Answer
How many bytes is the payload?
Can this packet be further segmented?
Which upper layer protocol has been used?
How many hops can this packet visit before being discarded?
Is this packet the last packet? Why?
What is the source ip address?

Answer 1

Payload size= (Total length of the packet)-(Header size)

2nd half of the first byte in the given packet indicates IHL field.

IHL=7

Header size = IHL*4 = 7*4 = 28 bytes

3rd and 4th bytes of ip packet indicates the Total length field.

Total length= 0x(0050)

TOTAL length=80 bytes

Payload=80-28= 52 bytes

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This packet can't be fragmented further.

Because MF is set to 1.

The upper 3 bits from the 7th byte is 0x60

and the corresponding value for these fields is (“IP header flag field”), 0x40 = 0110 0000, matching the upper 3 bits values to the order of the bit-fields in the IP flags field.

Bit 1 Reserved 0

Bit 2 Don't Fragment 1 <------- this indicates that this packet should not be fragmented.

Bit 3 More fragments 1 <----- this indicates that this is not the last packet. This field should 0 if this packet is the last one.

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The 9th byte is 0x03 and the corresponding value for this field is Time to live (TTL). This indicates maximum of 3 hops can be visited before discarded.

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The 13th, 14th, 15th and 16th bytes are “0xD8 3A C6 4E” and the corresponding value for these fields is (“Source Address”).

Source IP= 216.58.198.78

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