VLSM Calculator & Subnet Planning Guide
Variable Length Subnet Masking lets one parent network contain child subnets of different sizes. Use the calculator to verify CIDR blocks while planning an efficient VLSM allocation.
How VLSM allocation works
A typical VLSM plan sorts host requirements from largest to smallest, chooses the smallest subnet that can satisfy each requirement, then allocates those blocks without overlap.
For traditional IPv4 LANs, remember to account for network and broadcast addresses when sizing the block. A requirement for 50 hosts typically needs a /26 with 64 total addresses and 62 conventional usable hosts.
Example VLSM sizing
Suppose departments need 100, 50, 20 and 10 conventional host addresses. Suitable sizes are /25 (126 usable), /26 (62 usable), /27 (30 usable) and /28 (14 usable). Allocating the largest block first helps maintain clean alignment.
Why VLSM saves address space
Fixed-length subnetting forces every subnet to be as large as the largest requirement. VLSM lets small networks use smaller prefixes, leaving more of the parent address block available for other allocations.
Avoiding overlap
Every allocation must begin on the correct boundary for its prefix and must not intersect an earlier block. After assigning a subnet, advance to the next address after that block before placing the next requirement.
What this calculator shows
The result panel reports the canonical network address, broadcast address, first and last conventional usable addresses, dotted-decimal subnet mask, wildcard mask, total IPv4 address count and usable-host count. The live 32-bit boundary updates with the CIDR prefix, so the relationship between prefix length, block size and address capacity is visible instead of hidden behind a static table.
How to read the result correctly
The calculator normalizes the IP address you enter to the network block that contains it. That means an input such as 192.168.1.42/24 correctly resolves to network 192.168.1.0/24. For prefixes /0 through /30, conventional IPv4 subnetting reserves the first address as the network identifier and the final address as broadcast. /31 and /32 have different practical semantics, so their address counts should not be interpreted with the ordinary “subtract two” rule.
Related subnet tools
You can also use the subnetting cheat sheet for a complete IPv4 prefix reference, browse individual CIDR prefix pages, try the IPv6 subnet calculator, or return to the main visual subnet calculator.
Frequently asked questions
What is VLSM?
VLSM stands for Variable Length Subnet Masking. It allows different-size subnets to coexist inside a larger address block.
How do I choose a VLSM prefix for 50 hosts?
A conventional IPv4 network needing 50 hosts normally uses /26, which has 64 total addresses and 62 usable hosts.
Should I allocate large VLSM subnets first?
Yes. Sorting requirements from largest to smallest generally makes boundary alignment easier and reduces fragmentation.