Computer Science - Networking - Discussion

Step 1: Determine how many bits you need for subnets.

We're starting with a Class C network:

Default subnet mask = 255.255.255.0 → 8 bits available for hosts.

To get at least 9 subnets, we need to borrow bits from the host portion.

Borrowed Bits: Subnets Created
1 : 2
2: 4
3: 8
4: 16 ✅

So, we borrow 4 bits, leaving 4 bits for hosts.

Step 2: Determine the number of hosts per subnet;
With 4 host bits left:

Number of hosts per subnet = 2^4 -2 = 14.
Why subtract 2? One address is reserved for the network and one for the broadcast.

Step 3: Choose the correct subnet mask
Borrowed 4 bits → New subnet mask = /28.

CIDR /28 corresponds to: 255.255.255.240.

2^1 = 2, therefore if 1 or 2 subnet is required then we require 1 bit, so 10000000, likewise now in this question we require 9 subnets, as 8<9<16 we will have to use 16, which is 2^4 and hence 11110000, which is 240.

Explain it clearly.

C is the right answer because we need 4 bits for 9 host IDs. So we will pick first four bits from left which will be like this.

128+64+32+16 = 240. So C is right option.