How to use the VLSM Calculator
- 1. Enter the networkThe block you're allowed to use, such as 192.168.1.0/24.
- 2. List the subnetsOne per line: a name and the number of hosts it needs.
- 3. Plan subnetsGet each subnet's network, mask, usable range and broadcast, and download the plan.
How VLSM allocation works
Variable Length Subnet Masking gives each subnet only the space it needs, instead of cutting a network into equal pieces. For each subnet the tool adds 2 to the hosts needed (for the network and broadcast addresses) and rounds up to a power of two: 100 hosts need 128 addresses, a /25. Subnets are placed largest first, each starting on a boundary of its own size, which is what keeps them from overlapping and leaves the free space in one block at the end. If the network is too small, the tool tells you where it runs out. Everything is calculated in your browser; nothing you enter is uploaded.
Worked example
Dividing 192.168.1.0/24 for a college: students lab 100 hosts, staff 50, accounts 25, servers 10, and two router links of 2 hosts each. The plan is 192.168.1.0/25 for the lab (126 usable), 192.168.1.128/26 for staff, 192.168.1.192/27 for accounts, 192.168.1.224/28 for servers, and 192.168.1.240/30 and 192.168.1.244/30 for the links. That uses 248 of 256 addresses (96.9%), with 192.168.1.248 onwards still free.
Tips
- Plan for growth: ask for the hosts you'll need in two or three years, not today.
- Keep router links as /30 (or /31 where your equipment supports it).
- We checked the planner against an independent allocator on 150 random plans, including ones that can't fit.
Frequently asked questions
Why largest first?
Placing big subnets first keeps every block aligned and avoids gaps that small subnets would leave.
Can I plan IPv6 this way?
IPv6 is usually split into /64s, so VLSM isn't needed; use the subnet calculator's split option instead.
Is the VLSM calculator free?
Yes. You can open the VLSM calculator and use every option without signing up. When you plan subnets for the first time, we ask once for your name and WhatsApp number; after that it, and every other Uncodemy tool, opens straight away.