Another Try
Let’s first review breaking a network number down with a subnet requirement:
Requirement: A site has been assigned the network number 150.1.0.0. It requires 100 hosts per subnet. Future growth indicates 120 hosts per subnet. It was determined that expansion was more likely in the case of remote sites than hosts.
Step 1: Determine the bits required to support at least 100 hosts and future expansion to 120 hosts per subnet.
7 bits are required for 100–126 hosts. Start from the right and move left.
Step 2: Determine how subnets are defined by 9 bits.
9 bits support 512 subnets. Start from the left and move right.
Step 3: Determine the mask.
150.1.0.0/25, or 255.255.255.128