Strategies to Conserve Addresses 95 The custom subnet mask, also called a variable length subnet mask, is used to identify the bits used for a network address versus the bits used for a host address. Custom subnet masks are used when subnetting or supernetting. To determine the appropriate custom subnet mask, typically referred to simply as subnet mask, for a network, you must follow these steps: 1. Determine the number of host bits to be used for subnetting. a. Determine the maximum number of subnets required including consideration for future anticipated growth. b. Determine how many host bits are required to create the number of subnets. c. Add together the values of the left-most bits from the octets, yielding the highest network ID. 2. Determine the new subnetted network IDs. a. List all the possible binary combinations of the bits taken from the host address space. b. Calculate the incremental value to each subnet and add to the network address.