GATE CS 2012 Q34 — Fragmentation and IP Addressing, IPv4, CIDR

The ISP has the CIDR block $245.248.128.0/20$. The third octet starts at $128$ ($10000000_2$). With a $/20$ prefix, the first $4$ bits of the third octet are fixed as $1000_2$, giving a range of third octet values from $128$ to $143$ (total $16$ blocks of size $256$).

We need to allocate:

• Organization A: Half of the block $\implies$ needs a $/21$ subnet (size $8$ in the third octet).
• Organization B: A quarter of the block $\implies$ needs a $/22$ subnet (size $4$ in the third octet).

We can split the $/20$ block into two equal $/21$ subnets:

1. $245.248.128.0/21$ (covers third octet range $[128, 135]$)
2. $245.248.136.0/21$ (covers third octet range $[136, 143]$)

If we allocate the second half, $245.248.136.0/21$, to Organization A, we can further split the first half ($245.248.128.0/21$) into two $/22$ subnets:

• $245.248.128.0/22$ (covers third octet range $[128, 131]$)
• $245.248.132.0/22$ (covers third octet range $[132, 135]$)

Allocating $245.248.128.0/22$ to Organization B ensures there is no overlap between A and B, and both allocations are valid. This matches Option A.