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CIDR Notation Calculator
A CIDR Notation Calculator turns an IPv4 or IPv6 prefix into an exact network range. CIDR, short for Classless Inter-Domain Routing, writes an address followed by a slash and prefix length, such as 10.0.0.0/16 or 2001:db8::/48. The prefix length states how many leftmost bits identify the network; every remaining bit identifies addresses inside that block.
This calculator canonicalizes host bits, calculates exact boundaries, checks containment and overlap, splits a parent prefix into equal children, and summarizes compatible blocks without adding gaps. It also separates universal CIDR mathematics from AWS, Azure, and Google Cloud deployment rules.
How CIDR Notation Works
IPv4 addresses contain 32 bits and IPv6 addresses contain 128 bits. A longer prefix leaves fewer host bits and therefore creates a smaller network. An IPv4 /24 leaves eight host bits, producing exactly 256 addresses. An IPv6 /64 leaves 64 host bits, producing exactly 18,446,744,073,709,551,616 addresses.
A valid CIDR block starts on a boundary aligned to its size. When an input such as 192.168.1.42/24 includes nonzero host bits, its canonical network is 192.168.1.0/24. Canonicalization does not move the address into a different network; it reveals the actual network prefix represented by the notation.
CIDR to Subnet Mask Reference
| CIDR | Subnet mask | Total addresses | Traditional usable |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /20 | 255.255.240.0 | 4,096 | 4,094 |
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
| /31 | 255.255.255.254 | 2 | 2 on point-to-point links |
| /32 | 255.255.255.255 | 1 | 1 host route |
Traditional usable counts subtract the IPv4 network and broadcast addresses through `/30`. RFC 3021 permits both addresses in a `/31` on point-to-point links. Cloud platforms reserve addresses using their own rules.
IPv4 and IPv6 CIDR Differences
IPv4 uses 32-bit addresses, dotted-decimal subnet masks, and a broadcast address. IPv6 uses 128-bit addresses and prefix lengths only; it has no broadcast address. IPv6 `/64` is the standard subnet size in many environments, while a `/128` identifies one address.
CIDR Containment and Overlap
Containment asks whether every address in a child block belongs to a parent. Overlap asks whether two blocks share any address. Avoiding overlap is essential when planning VPC peering, VPN routes, firewall policies, and private network mergers.
Splitting CIDR Blocks
Equal splitting lengthens the prefix. Splitting a `/24` into `/26` networks adds two prefix bits, creating four aligned children with 64 addresses each. A requested number of equal children must be a power of two because every CIDR block has a power-of-two size.
Summarizing CIDR Blocks
Summarization reduces route entries by merging exact sibling prefixes. Two aligned `/25` blocks can become one `/24`. Adjacent blocks that are not aligned siblings cannot be merged safely because the parent would include addresses absent from the original set.
AWS, Azure, and Google Cloud CIDR Planning
A mathematically valid CIDR prefix is not automatically deployable on every cloud platform. AWS VPC, Azure Virtual Network, and Google Cloud VPC enforce different subnet-size limits and reserve different addresses. The deployment profile in this calculator evaluates reviewed general rules without changing the underlying network boundaries.
Plan enough spare address space before deployment. Expanding or replacing a production subnet can require migrations, route updates, and security-policy changes. Also check whether planned networks overlap with on-premises ranges, partner networks, VPN destinations, and other cloud environments.
Private IP Address Ranges
| Range | Purpose | Notes |
|---|---|---|
| 10.0.0.0/8 | Private IPv4 | Large private network range |
| 172.16.0.0/12 | Private IPv4 | 172.16.0.0 through 172.31.255.255 |
| 192.168.0.0/16 | Private IPv4 | Common home and small-network range |
| 100.64.0.0/10 | Shared address space | Carrier-grade NAT; not RFC 1918 private space |
| fc00::/7 | IPv6 unique local | Locally assigned IPv6 address space |
CIDR Calculator FAQ
What does /24 mean in CIDR notation?
A /24 means the first 24 bits identify the network and the remaining eight bits identify addresses inside it. In IPv4, that creates 256 total addresses and corresponds to subnet mask 255.255.255.0.
How many IP addresses are in a /24 network?
An IPv4 /24 contains exactly 256 addresses. Traditional subnetting treats 254 as usable host addresses, although cloud-provider reservations can reduce the assignable count further.
How do I convert CIDR notation to a subnet mask?
Set the number of leading one-bits equal to the prefix length, then set every remaining bit to zero. A /26 becomes 11111111.11111111.11111111.11000000, or 255.255.255.192.
How can I tell whether two CIDR blocks overlap?
Two same-family CIDR blocks overlap when either block contains the other's canonical network address. IPv4 and IPv6 blocks never overlap because they belong to different address families.
What is the difference between a subnet and a CIDR block?
A CIDR block is a mathematically defined contiguous address range. A subnet is a network segment created from an address range and can have platform-specific routing, reservation, and deployment rules.
Why was my CIDR input changed to a different network address?
The entered address contained host bits. CIDR canonicalization clears those host bits to reveal the actual network boundary represented by the prefix length.
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