This subnet mask will allow you to have six usable IP addresses in each subnet, with a minimum of wasted addresses. You can use the following formula to calculate the number of subnets and the subnet ID for each subnet:
Number of subnets = 2^network bits Subnet ID = (subnet number - 1) x number of host addresses + network address The network bits are the number of 1s in the binary representation of the subnet mask, and the network address is the first address in the range. For example, if your range is 192.168.1.0/27, then the network bits are 27, the network address is 192.168.1.0, and the number of host addresses is 30. Therefore, the number of subnets is 2^27, and the subnet ID for the first subnet is (1 - 1) x 30 + 192.168.1.0 = 192.168.1.0. The subnet ID for the second subnet is (2 - 1) x 30 + 192.168.1.0 = 192.168.1.32, and so on.
Reference
Subnet masks are covered in Objective 1.4 of the CompTIA Network+ N10-008 certification exam1.
Subnet masks can be calculated based on binary and CIDR-block notations2.
Subnet masks can be used to determine the number of host bits and host addresses3.
1: CompTIA Network+ Certification Exam Objectives, page 4 2: IPv4 Subnet Masks - N10-008 CompTIA Network+ : 1.41 3: Calculating IPv4 Subnets and Hosts - N10-008 CompTIA Network+ : 1.44