Subnet planner
Three jobs the subnet calculator does not do: split one network into subnets of different sizes, turn an address range into the CIDR blocks that cover it, and merge a list of blocks into the fewest.
Split a network
List the subnets you need and how many hosts each must hold. The largest are placed first, which keeps every block aligned with no gaps.
Range to CIDR
The exact CIDR blocks that cover a range of addresses, for firewall rules and allow lists that only accept CIDR.
Merge CIDR blocks
Paste blocks or single addresses. Overlapping and adjacent ones are combined into the fewest blocks that cover exactly the same addresses.
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Leave room to grow
Plan each subnet for the hosts it will have in two or three years, not today. Renumbering a live network is far more work than a slightly larger block, and the free space listed after a plan is where the next subnet goes.
Where range conversion and merging help
- Firewall and cloud security rules often accept only CIDR, while a vendor documents a range. Convert it rather than rounding it to a larger block.
- Allow lists grow by accretion. Merging collapses years of added blocks into the fewest entries, which matters where rule counts are limited.
- Checking a list — if merging leaves it unchanged, nothing in it overlaps.
The subnet calculator describes a single block in detail, and subnet masks in five minutes explains the arithmetic.
What is VLSM?
Variable-length subnet masking: cutting one network into subnets of different sizes, each just large enough for its hosts, instead of equal pieces. A 100-host office and a 2-address router link no longer take the same space.
Why does each subnet need two more addresses than its hosts?
The first address names the network and the last is the broadcast address; neither can be given to a device. So 100 hosts need a block of at least 102, which rounds up to 128 — a /25.
Why are the largest subnets placed first?
A block must start on a multiple of its own size. Placing blocks from largest to smallest keeps every one aligned with no gaps between them; the reverse order wastes space.
Can it plan IPv6?
There is nothing to plan by host count: every IPv6 LAN is a /64, whatever its size. The IPv6 calculator shows how many /64s an allocation holds, and merging and range conversion here work for IPv6 too.
Is anything I type sent anywhere?
No. All three tools run in your browser and make no requests.