Cisco Certified Network Associate (CCNA) Practice Exam

You need to apply a filter for IP subnets 172.16.0.0/24 to 172.16.15.0/24. What is the wildcard mask?

1. 0.0.0.255

2. 0.0.255.255

3. 0.0.15.255

4. 0.0.31.255

The correct answer is: 0.0.15.255

To determine the correct wildcard mask for the IP subnets ranging from 172.16.0.0/24 to 172.16.15.0/24, it's important to understand how the wildcard mask calculation works in relation to subnet masking. In this case, the source address range starts from 172.16.0.0 (which uses the subnet mask 255.255.255.0) and extends to 172.16.15.0, indicating that the first two octets (172.16) remain static while the third and fourth octets can vary. Since the subnet portion being filtered (the first 24 bits) remains fixed, we need to establish which bits can vary. The IP addresses in binary reveal that the third octet ranges from 00000000 (0) to 00001111 (15 in decimal) and the fourth octet can range from 00000000 (0) to 11111111 (255 in decimal). To accurately represent this within a wildcard mask, the bits that remain static are 0 (which translates to the wildcard mask), while the variable bits (in this case, the last octet and the variable part of the third octet) must