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VLSM Calculator
Enter a base network and host requirements — get optimally sized VLSM subnets, largest first
Base Network
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Subnet Requirements
Sorted largest-first automatically (VLSM)
hosts
hosts
hosts
How VLSM works
Variable Length Subnet Masking allows you to divide an address space into subnets of different sizes, rather than splitting everything into equal-sized blocks. The rule is: always allocate the largest subnet first.
- Sort your requirements from most hosts needed to fewest.
- Find the smallest prefix that accommodates the host count (2^n − 2 ≥ hosts).
- Assign that block starting at the next available address.
- Move the pointer to the address after that block's broadcast, then repeat.
VLSM is tested directly on the CCNA 200-301 exam — expect a scenario question that gives you a /24 or /23 and asks you to allocate efficiently for several departments with different host counts.