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

Address space:
   199.16.7.0

   So, we have 4th(last) octet bits for subnetting and also for host configuration

   Since we need at leat 12 host to configure in each network after sunbetting, so we need at leat 4 bits for host id
   (size of network is always in power of 2)

   No since we have use network efficiently, so we can user remaining bits (4 bits) for subnetting

   So, Subnet mask without subnetting for 199.16.7.0
   = 255.255.255.0

   After using 4 bits for subnetting from last octet, subnet mask:
      255.255.255.11110000
      = 255.255.255.240

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