VLSM Subnet Calculator

Imagine you have a block of 256 IP addresses and three rooms to connect — one with 100 computers, one with 50, and one with just 10. With regular subnetting you would give each room the same amount of addresses, wasting hundreds. VLSM lets you give each room exactly what it needs — no more, no less — so almost nothing goes to waste.

First, define the total pool of addresses you have available to divide up. All subnets will be carved out of this block.

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 your chosen prefix (e.g. /24).
3. 3The prefix sets the total pool size. A /24 gives 256 addresses to divide, a /23 gives 512, and so on.

This is the key difference from FLSM. Instead of one number for all subnets, you type a separate number for each subnet you need — all in one box, separated by commas.

1. 1In Required Hosts per Subnet, type how many devices each subnet needs, separated by commas — e.g. 100, 50, 20 for three subnets.
2. 2The No. of Subnets field updates automatically — it counts how many values you have entered. You do not need to touch it.
3. 3The calculator automatically sorts values from largest to smallest before allocating — largest subnets must be placed first.

Click Calculate VLSM. Each row in the 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 address. 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 in the block. 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 setting up the network.
8. →Wildcard — Inverse of the netmask. Used in firewall rules and router access lists.

Example — splitting 192.168.0.0/24 for 100, 50, and 20 devices

After all your subnets are placed, addresses left over at the end of the original block appear in the Free/Unused IP Network table — with First Usable IP, Last Usable IP, broadcast, mask, and wildcard already calculated.

Continuing the example above — addresses left over after 3 subnets

VLSM is a method of dividing a block of IP addresses into subnets of different sizes. You decide how many devices each subnet needs and the calculator gives each one exactly the right amount of addresses. This is much more efficient than traditional subnetting where every subnet is forced to be the same size.

Total Usable Host is the number of addresses you can actually assign to devices — the block size minus 2 (network + broadcast). Total Available Host is the full block size including those two reserved addresses. For a /25: Total Usable Host = 126, Total Available Host = 128. For a /27: Total Usable Host = 30, Total Available Host = 32.

With FLSM every subnet is exactly the same size — if one area needs 100 devices and another only 5, FLSM still gives both the same large block, wasting addresses. VLSM sizes each subnet to match what it actually needs. Less waste, more flexibility.

VLSM places subnets one after another from the start of your address pool. The largest subnet must go first because it needs the most contiguous space. Sorting largest to smallest before allocating ensures every subnet slides into place without gaps or overlaps.

Once all subnets are placed, addresses left over at the end of your original block are shown in the Free/Unused IP Network table as a ready-to-use network entry — with exact range, mask, and wildcard already calculated.

If the total addresses needed exceed your base network size, the calculator shows an error. To fix it, lower the CIDR number to increase the pool (e.g. /24 → /23), or reduce some of the host counts you entered.