What FLSM is
FLSM stands for Fixed Length Subnet Mask. Every subnet created from the parent network uses the same prefix length and therefore the same address capacity. It is simple, predictable and easy to summarize, which makes it useful for training, homogeneous environments and designs where equal-size segments are acceptable.
The trade-off is efficiency. If one department needs 100 hosts and another needs only 10, an FLSM plan sized for the larger requirement gives both subnets the same block and wastes addresses in the smaller segment. The FLSM guide explains this design model in detail.
How the FLSM calculator works
Start with a parent IPv4 network and specify either the number of equal subnets you need or the host requirement that every subnet must satisfy, depending on the calculator mode. The tool determines how many host bits can be borrowed, selects the resulting prefix and lists the network boundaries in order.
Because every child subnet has the same prefix, the increment stays constant throughout the plan. That makes the table easy to verify manually with the subnet increment calculator.
Borrowing bits from the host portion
Creating more subnets means moving the prefix to the right and borrowing bits that previously belonged to hosts. Borrowing one bit creates twice as many equal blocks; borrowing two bits creates four times as many; borrowing three bits creates eight times as many. Each borrowed bit therefore doubles the number of subnets while halving the address capacity of each one.
For example, splitting a /24 into four equal networks requires two additional network bits, producing four /26 subnets. Their starts are .0, .64, .128 and .192.
Choosing host semantics
The calculator distinguishes traditional LAN capacity from point-to-point semantics. A /30 is the classic four-address point-to-point example with two traditionally assignable hosts, while RFC 3021 allows a /31 to connect two endpoints without reserving network and broadcast addresses. A /32 represents one address rather than a shared broadcast segment.
This distinction is important when your FLSM plan includes WAN links or loopback-style allocations instead of only user VLANs.
FLSM compared with VLSM
FLSM is easier to read because all child subnets share one prefix. VLSM uses different prefixes to match different requirements and usually reduces waste. If your requirements vary substantially, the VLSM calculator is normally the better planning tool.
Neither method changes the underlying CIDR rules. Both depend on correct prefix boundaries, address capacity and non-overlapping allocations.
For documentation, give every generated subnet a clear purpose instead of keeping only the numeric range. Recording the VLAN, gateway, site, expected host count and reserved growth space turns the mathematical result into an operational plan that another administrator can understand and verify later.
Frequently asked questions
When should I use FLSM?
Use it when equal-size subnets are desirable, when simplicity is more important than address efficiency, or when practicing fixed-prefix subnetting.
Can the number of requested subnets be any value?
The resulting binary subdivision occurs in powers of two. The calculator chooses enough borrowed bits to provide at least the required number of equal blocks.
Why can FLSM waste addresses?
Every subnet receives the same capacity even when some segments need far fewer addresses than others.
Does FLSM work with /31 links?
Yes when point-to-point semantics are explicitly intended. The calculator exposes the address-counting mode rather than assuming every subnet is a traditional LAN.
Validate the plan
Check that every child network remains inside the parent prefix, that consecutive ranges do not overlap, and that the chosen capacity covers the real requirement. For an independent check, compare selected rows with the network/broadcast calculator.
That documentation also makes later migration to VLSM easier.