FLSM Subnet Calculator

Imagine you have a block of 256 IP addresses and you need to create three separate network areas. With FLSM, every area gets exactly the same number of addresses — whether it has 5 devices or 50. It is the simplest way to split a network because every subnet uses the same mask and follows the same rules throughout.

First, tell the calculator which block of IP addresses you want to split up. Think of this as drawing a fence around the total number of addresses you have available before dividing them into equal groups.

1. 1Type your base IP Address — for example 192.168.0.0.
2. 2Set the Subnet Mask by typing it directly (e.g. 255.255.255.0) or drag the CIDR slider to the prefix you want (e.g. /24).
3. 3The prefix length sets the total pool size. A /24 gives you 256 addresses, a /23 gives 512, and so on.

Now tell the calculator how many equal groups you want to create and how many devices each group needs to support.

1. 1In No. of Subnets, type how many subnets you want — for example 3.
2. 2In Hosts per Subnet, type how many devices each subnet needs — for example 50. Enter a single number, not a list.
3. 3The calculator finds the right block size automatically. You do not need to work out the prefix yourself.

Click Calculate FLSM. Each row in the results table is one subnet. Here is what each column means:

1. →Subnet Name — A simple label (Subnet 1, Subnet 2 …) to tell your subnets apart.
2. →Network — The subnet identifier. Cannot be assigned to any device.
3. →First Usable IP / Last Usable IP — The first and last addresses you can assign to devices.
4. →Broadcast — Reserved last address. Cannot be assigned to a device.
5. →Total Usable Host — Assignable addresses in this subnet (block size − 2).
6. →Total Available Host — Full block size including network + broadcast addresses.
7. →Netmask — Enter this into your routers and devices when configuring the network.
8. →Wildcard — Inverse of the netmask. Used in firewall rules and router access lists.

Example — splitting 192.168.0.0/24 into 3 subnets of 50 devices each

After your subnets are created, the calculator checks whether any addresses are left over inside your original block. If there are, they appear automatically in the Free/Unused IP Network table below the subnet results.

Continuing the example — addresses left over after 3 subnets of 50 hosts

FLSM is a way of splitting a large block of IP addresses into smaller, equal-sized groups called subnets. Every subnet you create is exactly the same size and supports the same number of devices. It works best when every part of your network needs roughly the same number of connections.

Total Usable Host is the number of addresses you can actually assign to devices — the block size minus 2 (network address + broadcast address). Total Available Host is the full block size including those two reserved addresses. For a /26: Total Usable Host = 62, Total Available Host = 64.

With FLSM every subnet is the same size, even if some parts of your network only need a handful of devices — those unused addresses are wasted. VLSM lets you make each subnet a different size so you only use exactly what you need. FLSM is simpler; VLSM saves more addresses when your segments have very different sizes.

You enter how many devices each subnet needs to support. The calculator adds 2 to that number (network + broadcast), then picks the next block size that is a power of 2 and large enough to fit. For 50 devices it needs at least 52 addresses, so it picks 64, giving 62 usable addresses per subnet.

After building your subnets there are often addresses left over inside your original block. The calculator shows those leftover addresses in the Free/Unused IP Network table. Every free block uses the same prefix and host count as your allocated subnets — ready to use as a future subnet with no recalculation needed.

The subnets you asked for are too large to fit inside the address block you defined. To fix it: lower the CIDR number to increase the pool (e.g. /24 → /23), reduce the No. of Subnets, or lower the Hosts per Subnet value.

FLSM is the right choice when all your network areas need roughly the same number of devices, when you want the simplest possible setup, or when studying for a networking exam. If different areas have very different sizes, VLSM will make much better use of your available addresses.