GATE CS 2013 Q37 — Fragmentation and IP Addressing, IPv4, CIDR

To determine the position of the datagram and the byte range of its payload, we analyze the given IPv4 header fields:

1. Position of the Datagram ($M$ bit):

   The More Fragments ($M$) bit is $0$, which indicates that there are no more fragments following this one. Thus, this is the last fragment.

2. Header Length ($HLEN$):

   The $HLEN$ field is expressed in units of 4-byte words.

   $$\text{Header Size} = HLEN \times 4 = 10 \times 4 = 40\text{ bytes}$$
3. Payload Size:

   The Total Length field represents the combined length of the header and the payload.

   $$\text{Payload Size} = \text{Total Length} - \text{Header Size} = 400 - 40 = 360\text{ bytes}$$
4. First Byte Sequence Number (Fragment Offset):

   The Fragment Offset field is expressed in units of 8 bytes. It specifies the offset of the payload relative to the start of the original unfragmented payload.

   $$\text{First Byte} = \text{Fragment Offset} \times 8 = 300 \times 8 = 2400$$
5. Last Byte Sequence Number:

   Since the payload starts at byte $2400$ and has a size of $360$ bytes, the sequence numbers of the bytes range from $2400$ to:

   $$\text{Last Byte} = \text{First Byte} + \text{Payload Size} - 1 = 2400 + 360 - 1 = 2759$$

Therefore, the datagram is the last fragment, with the first and last bytes of the payload being $2400$ and $2759$ respectively.