Introduction
FLSM means Fixed Length Subnet Mask. Every child subnet uses the same prefix, so every subnet contains the same number of addresses. It is the most direct subnetting method and a useful way to learn binary boundaries before moving to VLSM.
What FLSM is
With FLSM, all generated subnets share one prefix length. If the parent network is divided into /26 subnets, every child is a /26. That makes planning simple, but it can waste addresses when some segments need far fewer hosts than others.
FLSM works especially well in exercises that ask for a fixed number of equal subnets, or in environments where capacity requirements are intentionally standardized.
When to use FLSM
- When every subnet needs approximately the same capacity.
- When the requirement explicitly asks for equal-size subnets.
- When simplicity and operational consistency matter more than address efficiency.
- As a learning step before variable-length subnetting.
Do not force FLSM onto a highly uneven design if address space matters. That is where VLSM is usually the better choice.
FLSM in 30 seconds
- Identify the parent network and prefix.
- Determine how many equal subnets or hosts per subnet are required.
- Choose a longer prefix that satisfies the requirement.
- Calculate the block size.
- List network boundaries from the beginning of the parent block.
- For each subnet, derive network, broadcast, and usable range.
- Confirm every child remains inside the parent network.
Extract the requirements first
Before calculating, decide whether the exercise is driven by number of subnets or by hosts per subnet. These are related but not identical questions.
If the requirement is number of subnets
Borrow enough host bits so that 2^borrowedBits is at least the number of required subnets.
If the requirement is hosts per subnet
Choose enough host bits for the required address capacity. In conventional LAN semantics, usable hosts are typically 2^hostBits - 2. Treat /31 and /32 according to their specific semantics instead of applying minus two blindly.
Choosing the child prefix
Suppose the parent is 192.168.50.0/24 and you need four equal subnets. Four subnets require two borrowed bits because 2^2 = 4. Add those two bits to the /24 parent prefix, giving /26.
Each /26 contains 64 total addresses. In a conventional LAN it provides 62 usable host addresses.
Quick IPv4 prefix reference
- /24 → 256 total addresses
- /25 → 128
- /26 → 64
- /27 → 32
- /28 → 16
- /29 → 8
- /30 → 4
- /31 → 2 addresses, commonly usable as two endpoints on point-to-point links
- /32 → one address / host route
Block size
The block size tells you where each subnet starts. In the interesting octet, calculate 256 - maskValue. A /26 mask is 255.255.255.192, so the block size is 64.
The network boundaries are therefore .0, .64, .128, and .192 in the last octet.
Worked example step by step
Divide 192.168.50.0/24 into four equal subnets.
- Parent: /24.
- Required subnets: 4.
- Borrowed bits: 2.
- Child prefix: /26.
- Block size: 64.
The result is:
192.168.50.0/26— broadcast .63, conventional hosts .1–.62.192.168.50.64/26— broadcast .127, hosts .65–.126.192.168.50.128/26— broadcast .191, hosts .129–.190.192.168.50.192/26— broadcast .255, hosts .193–.254.
How to validate an FLSM result
Every child must have the same prefix, no blocks may overlap, all child networks must sit inside the parent, and the next network must start exactly one block size after the previous network.
Use the SubnetMaster FLSM calculator to verify your manual result, especially when the interesting octet is not the last one.
Typical FLSM mistakes
- Borrowing too few bits for the requested subnet count.
- Using the original parent mask when calculating child ranges.
- Incrementing by host count instead of block size.
- Starting a subnet on a non-aligned boundary.
- Forgetting that equal-size allocation can waste addresses.
Planning beyond the exercise
In a real address plan, equal-size subnets can be attractive because documentation, ACLs, DHCP scopes, and troubleshooting all follow the same pattern. The trade-off is efficiency: a subnet sized for the largest segment may leave many unused addresses in smaller segments. Before choosing FLSM for production, compare the operational simplicity it gives you with the amount of address space it consumes.
FLSM also works best when future growth is predictable. If one segment is likely to grow much faster than the others, a variable-length design may avoid renumbering later. The calculation method is therefore only one part of the decision; topology, growth, summarization, and operational standards matter too.
From FLSM to VLSM
Once equal-size blocks feel automatic, continue with VLSM. The binary rules remain the same; the difference is that each requirement may use a different prefix.