Cisco Certified Network Associate (CCNA) Practice Exam

How many bits are borrowed from the subnet mask to create 40 subnets in a Class B network?

1. 4 bits

2. 5 bits

3. 6 bits

4. 7 bits

The correct answer is: 6 bits

To determine how many bits need to be borrowed from the subnet mask to create a specified number of subnets, you can use the formula 2^n ≥ number of subnets, where n is the number of bits borrowed. In this case, the requirement is to create at least 40 subnets. Calculating step by step: 1. Start with the number of subnets you want, which is 40. 2. Find the smallest value of n such that 2^n is greater than or equal to 40: - For n = 5: 2^5 = 32 (which is not enough) - For n = 6: 2^6 = 64 (which meets and exceeds the requirement) Thus, 6 bits must be borrowed from the subnet mask to create at least 40 subnets. This process involves expanding the number of available subnets by repurposing bits that would normally be used for host addressing. In a Class B network, the default subnet mask is /16, allowing for a total of 65,536 addresses (2^16). Borrowing 6 bits changes the mask to /22, which enables the creation of 64 subnets (2