Online Subnet Mask Calculator - Subnetting / VLSM Planning
Subnetting / Borrowed bits / Usable hosts
About Subnetting
Based on the number of subnets you need, this tool calculates how many bits to borrow from the host portion. For detailed IP calculations (network address, broadcast address, etc.) use the IP Calculator; to convert CIDR notation use the CIDR Converter.
FAQ
What is subnetting and why do it?
Subnetting divides one large network into multiple smaller ones. Its main goals are: reducing broadcast domains to improve efficiency, isolating networks between departments, using IP address space more efficiently, and improving security and manageability.
How do I work out how many subnets are needed?
Decide the number of subnets from your actual needs (e.g. by department or floor), enter that number, and this tool automatically calculates how many bits to borrow from the host portion and shows the new subnet mask and usable hosts per subnet.
What does “borrowing bits” for a subnet mask mean?
Borrowing means taking some bits from the IP host portion to use as subnet bits. Borrowing n bits yields 2^n subnets, but each subnet has fewer hosts. For example, borrowing 2 bits from /24 gives /26 — 4 subnets of 62 usable IPs each.
What is the difference between the subnet calculator and the IP calculator?
The subnet calculator focuses on subnetting planning, deriving the mask from the required number of subnets; the IP calculator focuses on computing the specific network address, broadcast address and other details for a given IP and mask. Used together they cover full network planning.
What is VLSM (Variable Length Subnet Masking)?
VLSM allows subnet masks of different lengths within the same network, so address space can be allocated flexibly to each subnet’s actual host requirement, avoiding wasted IPs. For example, a subnet needing 100 IPs can use /25 while one needing only 10 IPs can use /28.