VLSM Calculator - Variable-Length Subnet Planner for IPv4
VLSM Calculator
Split a network into right-sized subnets
Right-Sized Subnets
Each subnet gets the smallest block that holds its hosts plus the network and broadcast addresses.
Largest First
Blocks are placed biggest first so they stay aligned and waste the least space.
Names Supported
Label each subnet, such as "Sales: 50", and the plan keeps the names.
Full Privacy
Everything is calculated in your browser.
How VLSM Works
Variable-length subnet masking lets each subnet be a different size instead of splitting a network into equal parts. Sort the requirements from largest to smallest, give each the smallest power-of-two block that holds its hosts plus two (the network and broadcast addresses), and place the blocks one after another. For 192.168.0.0/24 with 50, 25, 10 and 2 hosts, that gives a /26, a /27, a /28 and a /30, using 116 of the 256 addresses and leaving the rest free for growth.
Leave room to grow: a subnet sized exactly for today's hosts will need renumbering later. A /30 holds two usable hosts, which suits point-to-point links; RFC 3021 allows /31 links, which this planner does not use. This tool plans IPv4 only, and the blocks it shows are contiguous from the start of your network.
Key Takeaways
- Fit check: Tells you when the plan does not fit the network.
- Spare hosts shown: See how many addresses each subnet has left over.
- IPv4 only: For IPv6, see the IPv6 subnet calculator.