Cisco Certified Network Associate (CCNA) Practice Exam

If a /20 subnet is applied to a Class B address, how many subnets would be created?

1. 4

2. 8

3. 16

4. 32

The correct answer is: 16

To understand how many subnets would be created when a /20 subnet mask is applied to a Class B address, it's important to first grasp the structure of IPv4 addresses and subnetting. A Class B address has a default subnet mask of /16 or 255.255.0.0. This means that in a Class B network, the first two octets (16 bits) are used for the network part, while the remaining two octets (16 bits) are traditionally used for host addresses. When a /20 subnet mask is applied, we are extending the subnet mask by 4 bits (from /16 to /20). This means that 20 bits are now used for the network part of the address, leaving 12 bits available for host addresses. To calculate the number of subnets created by borrowing bits from the host portion, we use the formula 2^n, where n is the number of bits borrowed. In this case, since we are borrowing 4 bits, the formula becomes 2^4, which equals 16. Therefore, the application of a /20 subnet mask to a Class B address results in the creation of 16 subnets. Each of these subnets can further host a