Subnetting Formulas
Subnetting formulas can make subnetting much easier. Memorize the following two formulas:
- 2y - 2 = # of usable subnets (where y is the number of bits borrowed)
- 2x - 2 = # of usable hosts per subnet (where x is the number of bits remaining in the host filed after borrowing)
Assume that you have the subnet mask 255.255.255.240 on your Class C network address of 199.4.10.0.
To determine the number of subnets created, you would use the first formula above.
Since 4 bits are borrowed, the last significant binary digit is in the 16 position. Therefore the subnet numbers will increment by 16.