Here is an example question : Class C, Five Subnets. the book states that always look in the Y column. ofcourse the closest value for 5 subnets is 6.

This places the network in the Class C range and also fixes the length of the network address at 24 bits. To subnet this network, more than 24 bits must be set to 1 on the left side of the subnet ...

Class B subnets In my last Daily Drill Down, we borrowed bits from the host portion of a Class C network address. Class C network addresses only have eight bits to manipulate into subnets.

The more hosts, the fewer subnets; the more subnets, the fewer hosts can be individually addressed. These bits become a tradeoff based on the network class (A, B or C).