🔀 CIDR Splitter – Split IP Networks into Subnets
The CIDR Splitter is a network planning tool that divides a parent IP address block (in CIDR notation) into a set of smaller, equal-sized child subnets. Whether you're allocating address space for VLANs, cloud infrastructure, or branch offices, this tool instantly enumerates every resulting subnet with its network address, broadcast address, usable host range, and host count.
What is CIDR Splitting?
CIDR (Classless Inter-Domain Routing) is the standard notation for IP networks. A CIDR block like 192.168.0.0/24 represents 256 addresses. Splitting it means borrowing bits from the host portion to create sub-divisions — for example, splitting a /24 into four /26 subnets, each with 64 addresses and 62 usable hosts.
This process is fundamental to IP address planning, VLAN design, cloud subnet provisioning, and firewall rule simplification. Instead of managing one large block, you segment it into smaller, purpose-built ranges that map cleanly to network segments.
How to Use the CIDR Splitter
1. Enter the parent network – Type a valid IPv4 CIDR block, such as 10.0.0.0/22. The tool automatically corrects host bits (e.g., 10.0.0.1/22 becomes 10.0.0.0/22).
2. Choose a split method – Select By Prefix Length to specify the exact size of each child subnet (e.g., /26), or By Count to specify how many equal subnets you need (must be a power of 2).
3. Click Calculate– The tool enumerates all resulting subnets in a table, showing every subnet's CIDR, network address, broadcast address, first and last usable hosts, and usable host count.
4. Export or copy – Copy individual subnet CIDRs or download the full table as CSV for documentation.
Split Methods Explained
Split by New Prefix Length
Specify the prefix length each child subnet should have. The prefix must be strictly greater than the parent prefix. For example, splitting 10.0.0.0/22 by /24 produces exactly 4 subnets of 256 addresses each:
| # | CIDR | Network | Broadcast | Usable Hosts |
|---|---|---|---|---|
| 1 | 10.0.0.0/24 | 10.0.0.0 | 10.0.0.255 | 254 |
| 2 | 10.0.1.0/24 | 10.0.1.0 | 10.0.1.255 | 254 |
| 3 | 10.0.2.0/24 | 10.0.2.0 | 10.0.2.255 | 254 |
| 4 | 10.0.3.0/24 | 10.0.3.0 | 10.0.3.255 | 254 |
Split by Count
Specify how many equal subnets you need. The count must be a power of 2 (1, 2, 4, 8, 16, …). The tool automatically calculates the required prefix length: newPrefix = parentPrefix + log₂(count). For example, splitting 192.168.1.0/24 into 4 subnets produces four /26 subnets with 62 usable hosts each.
Key Formulas
New Prefix from Count
newPrefix = parentPrefix + log₂(count)e.g., /24 split into 4 subnets → /24 + 2 = /26
Subnet Count from Prefix
count = 2^(newPrefix − parentPrefix)e.g., /22 split to /24 → 2^(24−22) = 4 subnets
Addresses per Subnet
size = 2^(32 − newPrefix)e.g., /26 → 2^(32−26) = 64 addresses
Usable Hosts per Subnet
usable = size − 2 (for /1 – /30)/31 = 2 usable (RFC 3021), /32 = 1 host route
Common Use Cases
VLAN Segmentation
Split a /20 allocation into sixteen /24 subnets — one per department or floor — for clean Layer-3 segmentation.
Cloud Subnet Planning
Divide a VPC /16 into equal /24 subnets mapped to availability zones or microservices.
Branch Office Allocation
Distribute a /22 across 4 branch offices by splitting into four /24blocks with identical host capacity.
Firewall Rule Simplification
Replace a list of host addresses with a precise subnet range to keep ACLs clean and maintainable.
Understanding the Output Fields
| Field | Description |
|---|---|
| CIDR | The subnet in slash notation (e.g., 192.168.1.0/26) |
| Network | First address in the subnet — identifies the subnet itself |
| Broadcast | Last address — used to reach all hosts in the subnet |
| First Host | First assignable IP address (network + 1) |
| Last Host | Last assignable IP address (broadcast − 1) |
| Usable Hosts | Total assignable addresses (size − 2, with RFC 3021 for /31) |
| Subnet Mask | Dotted-decimal mask (e.g., 255.255.255.192 for /26) |
| Wildcard Mask | Inverse mask for Cisco ACLs (e.g., 0.0.0.63 for /26) |
Special Cases and Validation
The tool enforces several validation rules to ensure correct results:
- The parent network address must have all host bits set to zero. If you enter a host address (e.g.,
192.168.1.5/24), the tool auto-corrects it to the network address (192.168.1.0/24) with a visible notice. - The new prefix length must be strictly greater than the parent prefix. You cannot "split" a network into a larger or equal block.
- IPv4 prefixes are constrained to the range
/0–/32. - For performance, results above 256 subnets are paginated. The full list can always be exported as CSV.
/31subnets follow RFC 3021 (point-to-point links): both addresses are usable with no dedicated broadcast address./32is a host route: it has one address and zero traditional network/broadcast separation.
/8 into /24 subnets produces 65,536 results. The tool caps rendering at 65,536 subnets and paginates above 256 to maintain browser performance. Use the CSV export for very large splits.