Using a Class C address space of 19988770 you are asked to c

Using a Class C address space of 199.88.77.0, you are asked to create a subnet mask for the new network design. Each subnet must be capable of supporting 20 hosts. Which subnet mask best complies with the request, and how many subnets can be created?

A. 255.255.255.192, creating four subnets

B. 255.255.255.224, creating eight subnets

C. 255.255.255.240, creating 16 subnets

D. 255.255.255.248, creating 32 subnets

Solution

B is correct. To support at least 20 hosts per subnet, you must leave at least 5 bits (2ⁿ – 2 greater than or equal to the hosts supported), meaning you can only borrow 3 bits to create subnets with. With 3 bits, you can create eight subnets, with each subnet supporting up to 30 hosts. 255.255.255.224 is the correct subnet mask.

A, C, and D are incorrect. A is not correct because 255.255.255.192 will create four subnets, but leaves too many bits in the host field (6). C and D are not correct because neither choice leaves enough bits in the host field to comply with the scenario.