Question

An ISP has CIDR block 118.232.0.0/13. The ISP wants to create 19 sub-blocks from this block.

a. How many bits should the ISP add to the mask for the sub-blocks?
b. What are the first address and mask (include slash notation) for the first 2 sub-blocks?
c. What is the maximum number of hosts in each sub-block?
d. What if we subnet each of these sub-blocks into an additional 8 blocks? Show the first 2 new sub-blocks in dotted decimal.

Answer 1

To create 19 sub-blocks from the given CIDR block, the ISP needs to add 5 additional bits to the mask. The first 2 sub-blocks have addresses 118.232.0.0/18 and 118.232.64.0/18. Each sub-block can have a maximum of 16382 hosts. If each sub-block is further subnetted into 8 blocks, the first 2 new sub-blocks have addresses 118.232.0.0/21 and 118.232.8.0/21.

Step-by-step explanation:

a. To create 19 sub-blocks from a CIDR block, the ISP needs to add 5 additional bits to the mask. This is because 2^5 = 32, which is the minimum number of sub-blocks required to accommodate 19 sub-blocks.

b. The first address and mask for the first 2 sub-blocks would be:

• Sub-block 1: Address: 118.232.0.0/18
• Sub-block 2: Address: 118.232.64.0/18

c. The maximum number of hosts in each sub-block can be calculated using the formula 2^(32-mask) - 2. In this case, since the mask is /18, the maximum number of hosts in each sub-block would be 2^(32-18) - 2 = 16382.

d. If each of these sub-blocks is further subnetted into an additional 8 blocks, the first 2 new sub-blocks in dotted decimal would be:

• Sub-block 1.1: Address: 118.232.0.0/21
• Sub-block 1.2: Address: 118.232.8.0/21