About this tool
Variable Length Subnet Masking lets you divide a larger IPv4 block into differently sized subnets instead of forcing every segment to use the same mask. That reduces wasted address space when one segment needs hundreds of addresses and another needs only a few.
This tool sorts requested host counts from largest to smallest, chooses the smallest subnet that satisfies each request, and allocates each block sequentially from the parent network. That approach mirrors the way VLSM plans are commonly built by hand.
How to use it
- Enter the parent CIDR block, such as 10.10.0.0/22.
- Enter one host requirement per line or separate them with commas.
- Calculate and review the assigned subnet, prefix, usable range and capacity for each requirement.
Example: /22 with 120, 60, 30 and 12 hosts
The 120-host segment needs a /25, 60 needs a /26, 30 needs a /27, and 12 needs a /28. Allocating the largest first reduces fragmentation and leaves the remaining address space easier to use later.
Engineering notes
- Always leave growth room when the business case allows it. A subnet that is technically large enough today may be expensive to renumber later.
- Document reserved blocks and infrastructure ranges so an automated VLSM plan does not collide with addresses already committed elsewhere.
Frequently asked questions
Why are host requests sorted largest first?
Placing the largest networks first reduces the chance that small allocations fragment the parent block and make a larger request impossible later.
Does VLSM require a specific routing protocol?
No. VLSM is an addressing method. Modern classless routing protocols such as OSPF, IS-IS and BGP carry prefix length information.
Should I use every available address?
Not necessarily. Good plans usually reserve space for growth, infrastructure, loopbacks, point-to-point links and future services.