GATE CS 2016 Set 1 Q53 — Fragmentation and IP Addressing, IPv4, CIDR

To find the number of fragments, we need to separate the IP header from the data payload for both the original datagram and the fragments.

1. Analyze the Original Datagram

• Total Size: $1000$ bytes
• Header Size: $20$ bytes
• Payload Size: $1000 - 20 = 980$ bytes

2. Analyze the Fragment Constraints

• Link MTU (Maximum Transmission Unit): $100$ bytes
• Fragment Header Size: $20$ bytes
• Maximum Fragment Payload: $100 - 20 = 80$ bytes

The payload size of any fragment (except the last one) must be a multiple of $8$ bytes because the Fragment Offset field in the IP header is measured in units of $8$ bytes.

Since $80$ is a multiple of $8$ ($80 = 8 \times 10$), the maximum payload size per fragment is exactly $80$ bytes.

3. Calculate the Number of Fragments

The total payload of $980$ bytes is divided into fragments of maximum size $80$ bytes:

Thus, the IP datagram will be divided into $13$ fragments (where the first $12$ fragments carry $80$ bytes of payload each, and the last fragment carries the remaining $20$ bytes of payload).