Divide the address space.
Plan the subnet.

Read an IPv4 address and prefix

IPv4 addresses contain 32 bits, commonly written as four decimal octets. The subnet mask or prefix length identifies which bits describe the network and which identify the host. A /24 prefix uses 24 network bits and leaves 8 host bits. Network address, usable host addresses, and broadcast address are determined by the prefix.

Distinguish IPv4 delivery types

Unicast identifies one destination host. Broadcast targets all hosts in a local broadcast domain. Multicast targets a subscribed group; IPv4 multicast addresses are in 224.0.0.0/4 (224.0.0.0–239.255.255.255). Public IPv4 addresses are globally routable when allocated and routed. RFC 1918 private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. The 100.64.0.0/10 block is shared address space commonly used for carrier-grade NAT, while 169.254.0.0/16 is link-local/APIPA space. Private addresses are commonly translated at an Internet edge using NAT.

Subnet and size networks

Subnetting borrows host bits to create smaller networks. The source core task starts with a /24 and asks you to calculate smaller equal-size subnets. A /26 contains 64 total addresses and normally 62 usable host addresses; a /27 contains 32 total and 30 usable. VLSM uses different prefix lengths within an address block so each subnet can be sized for its host requirement. Allocate the largest subnet first, then continue with smaller aligned blocks. Calculations using larger parent prefixes such as /21–/23 and multi-size VLSM plans are useful advanced extensions, not a replacement for mastering the source /24 workflow.

Plan the subnetting scheme before assigning addresses

Start by listing the number of required subnets and the usable host count for each one. Decide which networks need private IPv4 space and whether any address block is shared or public. For equal-size subnetting, choose a prefix that creates enough subnets while preserving enough usable hosts in each. For VLSM, allocate the largest host requirement first and use correctly aligned, non-overlapping blocks for smaller networks. Record each network address, prefix/mask, usable range, broadcast address, default gateway, and any DHCP or VLAN address-pool requirement before configuring devices.

Calculate subnet count and host capacity

The prefix length is the number of leading network bits in the subnet mask. For example, /26 is 255.255.255.192, or 11111111.11111111.11111111.11000000 in binary. Count the contiguous 1 bits to read the prefix; the remaining 0 bits identify host positions.

For equal-size subnetting, borrowed bits = new prefix length − original prefix length, and the number of subnets is 2borrowed bits. Remaining host bits = 32 − prefix length; total addresses per subnet = 2host bits. For conventional subnets, usable host addresses are total addresses − 2 (network and broadcast). For example, splitting a /24 into /26 borrows 2 bits, producing 4 subnets. Each /26 has 6 host bits, 64 total addresses, and normally 62 usable host addresses. The block size is 64 in the final octet, so the subnet network addresses are .0, .64, .128, and .192.

The subtract-two rule is for conventional IPv4 subnets. Point-to-point /31 links and /32 host routes have special semantics.