IPv4 Subnet & CIDR Calculator

What is CIDR Notation and Why It Replaced Classful Addressing

In the early days of the internet, IP addresses were divided into rigid classes (Class A, B, and C). This classful addressing system was highly inefficient; an organization that needed 300 IP addresses had to be assigned a Class B network, which provided over 65,000 addresses, wasting tens of thousands of IPs. To solve this IPv4 exhaustion problem, Classless Inter-Domain Routing (CIDR) was introduced in 1993.

CIDR notation allows for variable-length subnet masking (VLSM). Instead of defaulting to an 8-bit, 16-bit, or 24-bit mask, network administrators can specify exactly how many bits of the IP address represent the network prefix. For example, a /23 prefix provides 512 total addresses (510 usable), perfectly fitting a medium-sized office network without the waste of a Class B assignment. This flexibility is the foundation of modern internet routing and cloud infrastructure design.

Subnet Mask vs CIDR: Two Ways to Express the Same Thing

A subnet mask and a CIDR prefix are simply two different ways of representing the boundary between the network and host portions of an IP address. A subnet mask uses a dotted-decimal format, such as 255.255.255.0. In binary, this mask consists of twenty-four 1s followed by eight 0s. The 1s lock in the network portion, while the 0s define the available host addresses.

CIDR notation is much more concise. Instead of writing out the full dotted-decimal mask, you simply count the number of 1s in the binary representation. Since 255.255.255.0 has 24 active bits, its CIDR equivalent is simply /24. When configuring SSH connections, firewalls, or routers, you will frequently encounter both formats. Our calculator bridges this gap by seamlessly accepting and converting between both notations.

Common Subnet Reference Table (/8 through /32 with host counts)

While our calculator handles any custom subnetting task, memorizing a few common CIDR blocks can dramatically speed up your workflow. Here are the most frequently used subnets in enterprise and cloud networking:

• /8 (255.0.0.0): 16,777,214 usable hosts. The default for a Class A network, often used for massive internal networks like 10.0.0.0/8.
• /16 (255.255.0.0): 65,534 usable hosts. The default for a Class B network, commonly used as the primary CIDR block for an AWS VPC.
• /24 (255.255.255.0): 254 usable hosts. The most common subnet size for home networks, small offices, and standard application tiers.
• /28 (255.255.255.240): 14 usable hosts. Frequently used for small, isolated infrastructure components like database clusters or load balancers.
• /31 (255.255.255.254): 2 usable hosts. A special subnet used exclusively for point-to-point links between two routers (RFC 3021).
• /32 (255.255.255.255): 1 host. Represents a single, specific machine. Often used in firewall rules or when testing a CORS configuration for a specific backend IP.

Planning AWS VPC Subnets Without Wasting IP Space

When designing an AWS Virtual Private Cloud (VPC), efficient IP allocation is critical. AWS reserves the first four IP addresses and the last IP address of every subnet for internal routing and DNS purposes. This means a /28 subnet, which normally provides 14 usable hosts, will only provide 11 usable hosts in AWS.

To prevent IP exhaustion, it is standard practice to create a large VPC (e.g., a /16) and divide it into smaller subnets using a Subnet Splitter. A common architecture involves splitting the VPC into public subnets (for load balancers and NAT gateways) and private subnets (for application servers and databases). For example, you might carve out several /24 subnets for your web tier while reserving larger /22 subnets for a rapidly scaling containerized application. Planning this hierarchy in advance prevents overlap and ensures you don't run out of IPs as your infrastructure grows. If you are integrating external data sources, you might also need a connection string parser to ensure your subnets can securely route to your managed databases.

How Wildcard Masks Work in ACLs and Routing

A wildcard mask is the mathematical inverse of a subnet mask. While a subnet mask uses 1s to define the network and 0s for the hosts, a wildcard mask uses 0s to indicate "must match" and 1s to indicate "can be anything." For example, the subnet mask 255.255.255.0 translates to the wildcard mask 0.0.0.255.

Wildcard masks are extensively used in Cisco Access Control Lists (ACLs) and routing protocols like OSPF. They allow network administrators to specify non-contiguous IP ranges, which is impossible with standard CIDR notation. For instance, a wildcard mask of 0.255.0.255 could be used to match all IPs that share the same first and third octets, regardless of the second and fourth octets. Understanding how to calculate both the subnet mask and its corresponding wildcard mask is essential for robust network security engineering.

Why Your Network Architecture Stays Private

Network topology data—including internal IP ranges, VPC architectures, and subnet splits—is highly sensitive information. Uploading your IP schema to a third-party server exposes your infrastructure to potential reconnaissance and mapping by malicious actors.

We built this Subnet Calculator to operate entirely within your browser. By utilizing pure, client-side JavaScript bitwise operations, all mathematical calculations and CIDR conversions happen locally on your machine. No IP addresses, subnet designs, or configuration values are ever transmitted over the network, logged to a database, or shared with analytics providers. Your network remains invisible, secure, and entirely under your control.