GATE CS 2025 Set 1 Q30 — Fragmentation and IP Addressing, IPv4, CIDR

To determine which interface the packet will be forwarded to, we apply the Longest Prefix Match (LPM) algorithm. We compare the destination IP address $145.36.109.70$ with each subnet entry in the routing table:

1. Subnet $145.36.0.0/16$ (Interface $E1$):
   The first 16 bits of the destination IP ($145.36$) match the subnet prefix.
   Match: Yes (Prefix length = 16).
2. Subnet $145.36.128.0/17$ (Interface $E2$):
   The third octet of the destination is $109 = 01101101_2$. The third octet of the subnet is $128 = 10000000_2$.
   For a $/17$ mask, the first bit of the third octet must match. Here, $0 \neq 1$.
   Match: No.
3. Subnet $145.36.64.0/18$ (Interface $E3$):
   The third octet of the destination is $109 = \mathbf{01}101101_2$. The third octet of the subnet is $64 = \mathbf{01}000000_2$.
   For a $/18$ mask, the first 2 bits of the third octet must match. Since both start with $\mathbf{01}$, they match.
   Match: Yes (Prefix length = 18).
4. Subnet $145.36.255.0/24$ (Interface $E4$):
   The third octet of the destination ($109$) does not match the subnet ($255$).
   Match: No.

Comparing the matching prefixes, the longest prefix match is $/18$ corresponding to interface $E3$.