CIDR Calculator
Expand CIDR blocks into IP ranges and split into smaller subnets.
Enter an IPv4 CIDR block (e.g., 10.0.0.0/16 or 172.16.0.0/12)
What Is CIDR?
CIDR (Classless Inter-Domain Routing) is the standard method for expressing IP address ranges. A CIDR block like 192.168.1.0/24 combines a network address with a prefix length — the number after the slash indicates how many bits define the network portion. CIDR replaced the older classful addressing system (Class A, B, C) in the mid-1990s, allowing much more flexible allocation of IP address space and significantly slowing the exhaustion of IPv4 addresses.
How CIDR Blocks Work
The prefix length determines the size of the network. A /24 block contains 256 addresses (2^8), a /16 contains 65,536 (2^16), and a /8 contains over 16 million addresses. Larger prefix numbers mean smaller networks. The network address is the first IP in the range (all host bits set to 0), and the broadcast address is the last (all host bits set to 1). Usable host addresses fall between these two — giving you 2^(32-prefix) − 2 usable addresses for prefixes /30 and smaller.
Splitting and Aggregating
Network engineers frequently need to split large CIDR blocks into smaller subnets or aggregate adjacent small blocks into a single larger one. For example, splitting a /24 into four /26 subnets gives you four isolated network segments with 62 usable addresses each — useful for VLAN segmentation, departmental isolation, or multi-tenant environments. This calculator shows you exactly which sub-ranges result from any split.
When to Use This Calculator
Use this tool when planning VPC subnets in AWS, Azure, or GCP; configuring firewall rules that reference CIDR ranges; allocating address space across multiple sites; verifying that two IP addresses belong to the same network; or auditing existing infrastructure to ensure address blocks don't overlap. The visual breakdown of ranges makes it easy to spot gaps or overlaps that are hard to catch with mental math.