A customer asks you to create a subnet mask for their networ

A customer asks you to create a subnet mask for their network, making sure address space is conserved as much as possible. Each subnet must support 12 hosts, and the address space given is 199.16.7.0. Which of the following answers best fits the customer’s needs?

A. 255.255.255.248, creating 14 subnets and providing 14 hosts per subnet

B. 255.255.255.240, creating 14 subnets and providing 14 hosts per subnet

C. 255.255.255.240, creating 16 subnets and providing 14 hosts per subnet

D. 255.255.255.224, creating eight subnets and providing 30 hosts per subnet

Solution

C is correct. 255.255.255.240 is the best choice, providing the best “subnets needed to hosts supported” range. The 240 mask creates 16 subnets, leaving 4 bits for up to 14 hosts.

A, B, and D are incorrect. A is not correct because 255.255.255.248 would not work since it only leaves 3 bits for hosts (six hosts on each subnet). B is not correct because 255.255.255.240 is the correct mask, but it creates 16 (2ⁿ) subnets, not 14. D is not correct because 255.255.255.224 would work in the scenario, creating enough subnets and supporting enough clients; however, it wastes quite a few addresses. The scenario called for only 12 hosts per subnet and conserving address space as much as possible. Therefore, only 4 bits (14 hosts) are needed in the host field, not 5 (30 hosts).