GATE CS 2023 Q55 — Fragmentation and IP Addressing, IPv4, CIDR

To determine the forwarding interface, we apply the Longest Prefix Match (LPM) rule. We perform a bitwise AND between the destination IP address $200.150.68.118$ and each subnet mask to see which entries match.

1. Interface 1: Mask $255.255.0.0$ (/16)
   $200.150.68.118 \text{ AND } 255.255.0.0 = 200.150.0.0$ (Match)
2. Interface 2: Mask $255.255.224.0$ (/19)
   $68 \text{ in binary} = 01000100_2$ and $224 \text{ in binary} = 11100000_2$.
   $68 \text{ AND } 224 = 64$.
   $200.150.68.118 \text{ AND } 255.255.224.0 = 200.150.64.0$ (Match)
3. Interface 3: Mask $255.255.255.0$ (/24)
   $200.150.68.118 \text{ AND } 255.255.255.0 = 200.150.68.0$ (Match)
4. Interface 4: Mask $255.255.255.224$ (/27)
   $118 \text{ in binary} = 01110110_2$ and $224 \text{ in binary} = 11100000_2$.
   $118 \text{ AND } 224 = 96$.
   $200.150.68.118 \text{ AND } 255.255.255.224 = 200.150.68.96 \neq 200.150.68.64$ (No Match)

Comparing the matching prefixes, the longest mask is $255.255.255.0$ (/24) corresponding to Interface 3.