An Internet Service Provider (ISP) has the following chunk of CIDR-based IP addresses available with it: $245.248.128.0/20$. The ISP wants to give half of this chunk of addresses to Organization A, and a quarter to Organization B, while retaining the remaining with itself. Which of the following is a valid allocation of addresses to A and B?

The ISP has the block $245.248.128.0/20$, containing a total of $2^{32-20} = 2^{12} = 4096$ IP addresses.

1. Organization A requires half of the chunk:

   $$\frac{4096}{2} = 2048\text{ addresses} = 2^{11} \implies /21\text{ prefix}$$

   The original $/20$ block can be split into two equal $/21$ subnets:

   • Subnet 1: $245.248.128.0/21$ (range $245.248.128.0$ to $245.248.135.255$)

   • Subnet 2: $245.248.136.0/21$ (range $245.248.136.0$ to $245.248.143.255$)

2. Organization B requires a quarter of the chunk:

   $$\frac{4096}{4} = 1024\text{ addresses} = 2^{10} \implies /22\text{ prefix}$$

If Organization A is allocated $245.248.136.0/21$, the remaining half ($245.248.128.0/21$) can be divided into two $/22$ subnets:

• $245.248.128.0/22$ (range $245.248.128.0$ to $245.248.131.255$)

• $245.248.132.0/22$ (range $245.248.132.0$ to $245.248.135.255$)

Organization B can be allocated $245.248.128.0/22$, and the remaining $/22$ block ($245.248.132.0/22$) is retained by the ISP. Both allocations are non-overlapping and valid.

Thus, (A) is the valid allocation.