Subnetting Worked Examples: Splitting a /24 Without the Headache

You have one /24 and three departments asking for addresses. Subnetting looks like binary-math gymnastics until you notice that almost every split comes down to one rule: the block size. Below are two complete worked examples — an even split into four subnets, then a split sized to what each department needs — plus three sanity checks. Every number here can be verified with the subnet calculator.

The block-size shortcut (forget binary math)

Every IPv4 address is 32 bits long. The prefix after the slash counts how many bits belong to the network; the rest belong to hosts. A block always contains a power-of-two number of addresses, and the sizes you need fit one short list: /25 blocks are 128 addresses, /26 are 64, /27 are 32, /28 are 16, and /30 are 4.

The shortcut: in the last octet, every network address must be a multiple of the block size. Carving /26s out of a /24 means each network ends in 0, 64, 128, or 192 — anything else, like 192.168.1.32/26, is not a network address at all. One rule replaces all the binary math.

Worked example: four equal subnets from one /24

Start with 192.168.1.0/24. Four equal subnets means borrowing two host bits, which moves the prefix from /24 to /26 and gives a block size of 64. Walk the last octet in steps of 64 and the four subnets write themselves.

Each subnet holds 64 addresses, 62 of them usable — the network and broadcast addresses are reserved. In dotted-decimal form, /26 is 255.255.255.192, and each broadcast address is one less than the next network address.

Worked example: fitting subnets to departments

Equal splits waste addresses. When requirements differ, allocate from the largest need downward, because every block must be a power of two. Take 10.10.0.0/24: Sales needs 100 hosts, Engineering 50, Guests 25, and a WAN link needs 2.

Sales needs 100 usable addresses, so the next power of two up is 128 — a /25 with 126 usable, taking 10.10.0.0/25. Engineering's 50 hosts fit a /26 (62 usable) at 10.10.0.128/26, the next multiple of 64. Guests need 25, so a /27 with 30 usable at 10.10.0.192/27. The WAN link takes a /30 — exactly two usable addresses — at 10.10.0.224/30. Everything from 10.10.0.228 up stays free for growth.

This technique — variable-length subnet masking, or VLSM — keeps a /24 from running out after the first two departments. Order matters: place the /25 last and it will not fit around the smaller blocks.

Checking your work: the three sanity checks

First, the multiples rule: every network address ends in a multiple of its block size. Second, broadcast arithmetic: each broadcast address is one less than the next network address, so overlapping or unexplained gaps mean a mistake. Third, capacity: usable hosts per subnet are 2^(32 − prefix) − 2, and your blocks must total no more than the 256 addresses you started with.

One practical habit remains: check the answer against the requirement — a department with 60 laptops does not fit in a /27. Once the numbers pass all three checks, paste them into your router config with confidence — and if you are working from a start and end range instead of a prefix, run them through the IP range to CIDR converter first.