Before you begin IPv4 subnetting, you must determine your organization’s current requirements and plan for future requirements. Follow these guidelines:
Determine how many subnets your network requires. Subnets include physical or logical subnets to which hosts connect and possibly private wide area network (WAN) links between sites.
Determine how many host IDs each subnet requires. Each host and router interface running IPv4 requires at least one IPv4 address.
Based on those requirements, you will define a set of subnetted address prefixes with a range of valid IPv4 addresses for each subnetted address prefix. Your subnets do not all need to have the same number of hosts; most IPv4 networks include subnets of various sizes.
Although the concept of subnetting by using host ID bits is straightforward, the actual mechanics of subnetting are a bit more complicated. Subnetting requires a three-step procedure:
Determine how many host bits to use for the subnetting.
Enumerate the new subnetted address prefixes.
Enumerate the range of IPv4 addresses for each new subnetted address prefix.