Variable length subnetting
| Subnet # | Required Hosts | Network | CIDR | Subnet Mask | First Host | Last Host | Broadcast | Usable IPs |
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Use VLSM whenever one network has subnets of different sizes. It assigns the smallest prefix that fits each requirement so you do not waste address space on a small VLAN.
Allocating biggest blocks first leaves the smaller ones to slot into the remaining space without overlap. If you allocate small blocks first they fragment the pool and large requests fail.
VLSM is allocation (splitting a supernet into sub-subnets of varying sizes). CIDR aggregation is the reverse: combining contiguous subnets back into a shorter prefix for advertisement.
Technically yes, but in practice IPv6 standardises on /64 LANs, /56 site allocations and /48 organisation allocations. VLSM is rarely needed because the address space is so large.
Enter a base network address with CIDR prefix and the number of hosts needed per subnet. The calculator applies Variable Length Subnet Masking to allocate the most efficient subnet sizes — largest subnets get the biggest blocks first. You'll see the network address, subnet mask, usable range, and broadcast for each resulting subnet.
VLSM prevents wasting IP addresses by letting you use different subnet masks within the same network. For example, a point-to-point WAN link needs only 2 IPs (/30) while a server VLAN may need 254 (/24). For standard single-size subnetting, use our Subnet Calculator. For IPv6 planning, try the IPv6 Subnet Calculator.