Cisco Certified Network Associate (CCNA) Practice Exam

What network mask should be used with a Class B network to create 40 subnets?

1. 255.255.255.0

2. 255.255.252.0

3. 255.255.0.0

4. 255.255.240.0

The correct answer is: 255.255.252.0

To determine the appropriate network mask for a Class B network that needs to create 40 subnets, it's essential to understand how subnetting works. Class B networks, by default, use a subnet mask of 255.255.0.0, which allows for a total of 65,536 addresses but doesn't add any subnets. To create subnets, you need to borrow bits from the host portion of the address. A Class B address has 16 bits allocated to the host portion, so by borrowing bits, you can create additional subnets. Each borrowed bit doubles the number of subnets available. Let's break down the calculation for 40 subnets: 1. Calculate how many bits are needed to support at least 40 subnets. The formula for calculating the number of subnets is 2^n, where n is the number of bits borrowed. - 2^5 = 32 (not enough) - 2^6 = 64 (sufficient) So, 6 bits need to be borrowed. 2. By borrowing 6 bits from the 16 host bits in a Class B address, the new subnet mask will be: - Original mask: 255.255