CIDR Split Calculator
Split larger CIDR blocks into smaller subnets with custom host counts.
CIDR Split Calculator
Runs entirely in your browser
This tool executes locally using standard browser APIs (for example window.crypto.getRandomValues or crypto.subtle where applicable). Your input is not transmitted to or stored on a ScriptPulse server. See How Calculations Work.
What This Tool Does
- The CIDR Split Calculator divides a single IPv4 CIDR block into smaller, right-sized subnets — either a specific number of equal-size blocks, or subnets sized to a minimum host count you specify per segment. Real network allocations are rarely used as one flat block: a /16 assigned to an organization typically needs to be carved into separate ranges for servers, workstations, IoT devices, and guest networks, each with different host-count requirements.
- Splitting is the inverse operation of aggregation (see the CIDR Aggregator): where aggregation combines contiguous blocks into one summary route, splitting takes one block and divides it along power-of-two boundaries into smaller, non-overlapping subnets. The math is mechanical (each additional network bit halves the available host count) but genuinely easy to get wrong by hand once you're juggling more than two or three target subnet sizes at once, especially when sizes aren't uniform.
- This tool computes each resulting subnet's network address, broadcast address, usable host range, and subnet mask, and verifies the requested splits actually fit inside the parent block without overlap — the exact verification step that's easiest to skip when planning subnets manually in a spreadsheet.
How It Works
- You provide a parent CIDR block (for example 10.20.0.0/16) and either a target prefix length for equal splits, or a list of required host counts for unequal splits.
- For equal splits, the tool extends the prefix by however many bits are needed to produce the requested number of subnets, and enumerates each resulting block in order.
- For host-count-based splits, the tool finds the smallest prefix length that satisfies each requested host count (accounting for the reserved network and broadcast addresses), then allocates blocks sequentially from the start of the parent range, always aligned to a valid power-of-two boundary.
- Each output block is checked against the others to confirm no two allocated ranges overlap and that none exceeds the parent block's boundary.
- Results are presented as a table: CIDR notation, subnet mask, network address, broadcast address, and usable host range, for each resulting subnet.
Usage
- Enter the parent CIDR block you've been allocated (for example 10.20.0.0/16).
- Choose whether to split into a fixed number of equal-size subnets, or specify individual host-count requirements per subnet.
- Run the split and review the resulting table of subnets, each with its network address, broadcast address, and usable host range.
- Cross-check the results against your actual device counts, then hand the table directly to whoever is configuring DHCP scopes or router interfaces.
- If a requested split doesn't fit inside the parent block, adjust the host-count requirements or the parent block size and re-run.
Examples
- Splitting 10.20.0.0/16 into a /24 for 250 servers, a /23 for 500 workstations, and a /26 for 60 IoT devices, then confirming none of the three overlap.
- Splitting a /24 evenly into four /26 subnets for four separate office floors.
- Sizing a subnet for exactly 30 required hosts and getting back the smallest valid block (a /27, which provides 30 usable addresses).
- Checking whether a /27 request actually fits in the remaining space of a /24 that already has two /26 subnets allocated.
Real-World Use Cases
- Carving a newly-assigned address block into separate ranges for servers, workstations, and IoT devices before provisioning DHCP scopes.
- Planning per-availability-zone or per-region subnet allocations inside a cloud VPC where each subnet needs a different size.
- Verifying that a proposed subnet plan actually fits inside an allocated parent block before submitting it for network-team review.
- Teaching subnetting mechanics by showing exactly how a block divides at different prefix lengths.
Best Practices
- Size each subnet for realistic future growth, not just today's device count — resizing a live subnet later means renumbering every device on it.
- Keep a single written record of every block you've allocated from a parent range, updated every time you run a new split, so two people don't independently allocate the same space.
- Prefer power-of-two-aligned subnet sizes even when a slightly smaller block would technically fit — non-aligned subnets are harder to further subdivide later and confuse most routing hardware's summarization logic.
- When splitting for a multi-region cloud VPC, leave at least one reserved block per region unallocated for future expansion rather than using the entire parent range immediately.
Common Mistakes
- Forgetting that the network and broadcast addresses in each resulting subnet are not usable host addresses — a /26 has 64 total addresses but only 62 usable hosts.
- Requesting subnet sizes that don't fit inside the parent block and not noticing until a router rejects the configuration — always let the tool's overlap/boundary check catch this before deploying.
- Manually splitting a block into unequal sizes without keeping them power-of-two aligned, producing subnets that can't be cleanly summarized later by the CIDR Aggregator.
- Assuming a single split calculation accounts for future growth — a plan sized exactly to today's device count with no headroom is the most common reason teams end up renumbering a subnet within a year.
Limitations
- This tool covers IPv4 splitting; for IPv6 prefix planning, use the IPv6 Subnet Calculator instead — the bit math is different enough (128 bits, nibble-aligned conventions) to warrant a separate tool.
- It calculates valid subnet boundaries but does not verify the split against your actual router or firewall configuration — always confirm the plan against real device configs before deploying.
- Extremely fragmented split requests (many different host-count sizes requested at once) may not produce the most space-efficient allocation order; for maximum space efficiency, sort your host-count requirements from largest to smallest before splitting.
Technical Reference Guide
- CIDR notation and variable-length subnet masking are defined by RFC 4632, which replaced the older classful A/B/C addressing scheme.
- Every subnet reserves exactly two addresses (network and broadcast) regardless of prefix length, except for /31 point-to-point links (RFC 3021) and /32 host routes, which have no broadcast address.
- Private IPv4 ranges commonly used for internal allocations are defined by RFC 1918: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
FAQ
How many usable hosts does a /27 provide?
A /27 has 32 total addresses, of which 30 are usable — the first address is the network address and the last is the broadcast address, neither assignable to a device.
Can I split a block into subnets of different sizes?
Yes — provide the host-count requirement for each subnet you need and the tool allocates a correctly-sized, non-overlapping block for each one from the parent range.
What happens if my requested subnets don't fit in the parent block?
The tool flags the overflow rather than silently producing an invalid plan — reduce the number or size of requested subnets, or start from a larger parent block.
Is splitting the same operation as subnetting?
Yes — splitting is the general term for dividing any CIDR block into smaller blocks; this tool specifically handles the case where you have several target subnets, possibly of different sizes, to carve from one parent block at once.
Why should subnets stay aligned to power-of-two boundaries?
Non-aligned subnets can't be cleanly represented in CIDR notation with a single prefix, complicate route summarization, and are rejected or misconfigured by some routing hardware — always let a split calculator produce aligned boundaries rather than picking arbitrary ranges by hand.
Can this tool reverse a previous split back into one block?
No — for combining multiple existing blocks back into the smallest possible summary route, use the CIDR Aggregator, which performs that inverse operation.
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