IP Subnet Calculator
Plan how to carve up an address block. Enter a network such as 192.168.1.0/24 and either split it into equal subnets of a chosen size, or list the departments, VLANs or links you need with their host counts and let the calculator assign each the smallest subnet that fits (VLSM), largest first, without gaps or overlaps.
- Runs in your browser
- No sign-up
- Free to use
One per line: a name and the number of hosts it needs.
How to use IP Subnet Calculator
- Enter the network you have, in CIDR notation.
- Choose equal subnets or hosts per subnet.
- Pick a subnet size, or list your subnets with host counts.
- Read the plan and download it as CSV.
IP Subnet Calculator features
Equal split
Every subnet of a chosen size within the network.
VLSM planning
Smallest fitting block for each requirement, allocated largest first.
Usable ranges
First and last host, broadcast address and spare capacity.
Fit check
Reports subnets that do not fit in the network.
CSV export
Take the plan into a spreadsheet or IPAM tool.
Private
Calculated in your browser.
When to use IP Subnet Calculator
- Designing VLANs for an office network.
- Allocating subnets in a cloud VPC.
- Studying for networking certifications.
- Checking whether a network has room for another segment.
IP Subnet Calculator FAQ
What is VLSM?
Variable-length subnet masking: giving each part of a network a subnet sized to its needs instead of using one size for all, which wastes fewer addresses.
Why are two addresses subtracted from every subnet?
The first address identifies the network and the last is the broadcast address; neither can be assigned to a host. (Point-to-point /31 links are an exception, but VLSM plans use /30 as the smallest block.)
Why are larger subnets allocated first?
Blocks must start on a multiple of their size. Allocating the largest first keeps every block aligned without leaving gaps.
Can I plan IPv6 subnets?
IPv6 networks are usually split into /64 subnets regardless of host count, so this planner focuses on IPv4. The CIDR Calculator handles IPv6 blocks.
Should I leave room for growth?
Yes. Enter host counts with a margin, or keep a spare block at the end of the network.
Is anything sent?
No.
Subnetting without the arithmetic
Subnetting divides one address block into smaller networks so that departments, VLANs or sites can be separated. Doing it by hand means binary arithmetic and careful alignment, and mistakes, overlapping ranges or blocks that do not start on a valid boundary, cause routing problems that are tedious to find.
Equal subnetting is the simple case: choose a size and the network is cut into equal pieces. Each additional bit in the prefix halves the subnet size and doubles the number of subnets. A /24 becomes four /26 networks of 62 hosts each, or eight /27 networks of 30 hosts.
Real networks rarely need equal pieces. An office segment may need a hundred addresses while a router link needs two. VLSM gives each the smallest block that fits. Sorting requirements from largest to smallest and allocating in that order guarantees every block lands on a correct boundary and the plan packs tightly.
A good plan leaves headroom, documents every allocation and avoids ranges used elsewhere, such as VPN pools or partner networks. Export the table and keep it with your network documentation; it is much easier to grow a network that was planned on paper first.