Carve a network into right-sized subnets. Enter a base network and each subnet's host count — get correctly-aligned CIDR blocks with masks and usable ranges, largest allocated first. The same engine that powers the Subnet Plus app, free on the web.
Subnet Plus keeps your VLSM plans in a synced library and adds the tools a browser can't run — DNS lookup, ping, and traceroute — on iPhone, iPad & Mac. Free, offline, iCloud-synced.
VLSM (Variable Length Subnet Masking) is the practice of using different subnet mask lengths within the same network so each subnet gets just enough addresses. Instead of splitting a network into equal pieces, you size each subnet to its host requirement, which conserves address space.
It sorts your required subnets from largest to smallest host count, picks the smallest prefix that fits each one (usable hosts = 2^(32 − prefix) − 2), and places them sequentially starting at the base network, aligning each block to its own size so subnets never overlap.
Allocating largest-first keeps every block aligned to its natural boundary and avoids fragmentation. If you placed small subnets first, a later large subnet might not find an aligned gap even though enough total space remains.
It means the sum of the aligned subnet blocks you requested exceeds the address space of the base network. Use a shorter base prefix (a larger network) or reduce the host counts to make everything fit.
Yes, it runs entirely in your browser with no signup. For VLSM plus DNS lookup, ping, and traceroute that need a real device — and to save and sync your plans over iCloud — download the free Subnet Plus app for iPhone, iPad, and Mac.
This free VLSM calculator runs entirely in your browser — nothing is sent to a server. Give it a base network like 10.0.0.0/24 and a list of subnets with the number of hosts each needs; it allocates the smallest CIDR block that fits every subnet, aligns each block to its own boundary so nothing overlaps, and flags when your requirements won't fit. It honors the /31 point-to-point case (RFC 3021) and /32 host routes, just like the app.
For a single network, use the subnet calculator; for a quick mask lookup, the CIDR cheat sheet; for IPv6, the IPv6 calculator. The Subnet Plus app adds DNS lookup, ping, and traceroute, plus a saved library and iCloud sync across iPhone, iPad, and Mac. It's been trusted by network engineers since 2013.