CIDR Calculator

About the CIDR Calculator

The CIDR Calculator turns a CIDR block into the subnet details you normally have to work out by hand: network address, broadcast address, usable host range, subnet mask, wildcard mask, and total host count. It is designed for the day-to-day subnetting tasks that come up in routing, cloud planning, labs, and certification study. That makes it useful when you want the network math, host range, and mask details in one place before you apply them to a real environment.

Enter an IP address and prefix length, such as 192.168.1.0/24, to see the resulting network breakdown. The tool shows where the subnet starts and ends, how many usable hosts it provides, and what the corresponding dotted-decimal mask looks like.

It also supports subnet splitting and supernetting, which makes it easier to reason about larger address plans and carve a network into smaller blocks. The prefix reference table gives you a quick way to compare common CIDR sizes, host counts, and subnet masks without recalculating everything each time.

When This Page Helps

Use this calculator to size subnets before you create VLANs, VPCs, ACLs, firewall rules, or lab environments. It keeps the host range, subnet mask, wildcard mask, and prefix length in one place, which reduces mistakes when you are planning or documenting an address layout. It is especially handy when you need to cross-check both host capacity and exact address boundaries quickly.

How to Use the Inputs

1. Enter an IP address (e.g., 192.168.1.100)
2. Enter or select the CIDR prefix length (e.g., /24)
3. Review the calculated network address, broadcast, and host range
4. Check the subnet mask and wildcard mask in dotted decimal format
5. View the number of available host addresses
6. Use presets for common network sizes
7. Reference the CIDR table for all prefix lengths

Example Calculation

Tips & Best Practices

• Always reserve the first and last addresses in a subnet for network and broadcast
• Use /30 subnets for point-to-point links (2 usable addresses)
• Common VPC sizes: /16 (65K hosts) for large deployments, /24 (254 hosts) for typical subnets
• Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
• Cloud providers often reserve additional addresses (e.g., AWS reserves 5 per subnet)
• Use the wildcard mask for Cisco ACLs and OSPF network statements

Understanding CIDR and Subnetting

CIDR was introduced in 1993 to replace the wasteful Class A/B/C IP address system. Instead of fixed 8/16/24-bit network portions, CIDR allows any prefix length from /0 to /32, enabling precise allocation of address space. This dramatically slowed the exhaustion of IPv4 addresses and is fundamental to modern networking.

Subnetting divides a network into smaller, more manageable pieces. A company with a /24 might create separate subnets for different departments: /26 for engineering (62 hosts), /27 for sales (30 hosts), and /28 for management (14 hosts). This improves security, reduces broadcast traffic, and makes network management easier.

Cloud Networking with CIDR

Cloud platforms like AWS, Azure, and GCP use CIDR extensively. When creating a VPC, you specify a CIDR block (typically /16 to /24). Within the VPC, subnets are created with smaller CIDR blocks. Understanding CIDR is essential for properly sizing networks, configuring security groups, and establishing VPN connections between cloud and on-premises networks.

IPv4 Address Exhaustion and IPv6

The IPv4 address space (2^32 = 4.3 billion addresses) is effectively exhausted. IPv6 with its 128-bit addresses provides a virtually unlimited supply. However, IPv4 remains dominant, and efficient subnetting through CIDR remains important for managing existing allocations.

Frequently Asked Questions

• What is CIDR notation?

  CIDR notation combines an IP address with a prefix length, such as 192.168.1.0/24. The prefix length tells you how many bits belong to the network portion, so the rest are available for hosts. That is why CIDR is so useful for both fixed-size and variable-size subnet plans.

• How do I calculate the number of hosts?

  Hosts = 2^(32 - prefix) - 2. The subtraction of 2 accounts for the network address and broadcast address, which cannot be assigned to hosts. For example, a /24 has 2^8 - 2 = 254 usable host addresses, while a /30 has only 2 usable addresses.

• What is a wildcard mask?

  A wildcard mask is the inverse of a subnet mask, and it is commonly used in ACLs and OSPF network statements. Where the subnet mask shows which bits are fixed, the wildcard mask shows which bits may vary. For 255.255.255.0, the wildcard mask is 0.0.0.255.

• What's the difference between /24 and 255.255.255.0?

  They describe the same subnet size in two different formats. /24 is CIDR notation, which says 24 bits are reserved for the network, while 255.255.255.0 is the dotted-decimal subnet mask with 24 leading 1s. CIDR is shorter and easier to aggregate, while dotted-decimal masks are still common in older documentation and device interfaces.

• What are private IP address ranges?

  10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are the RFC 1918 private ranges. They are reserved for internal networks and are not routable on the public internet. That makes them the standard choice for offices, home networks, labs, and cloud VPCs.

• How do I split a /24 into smaller subnets?

  A /24 can be split into 2× /25, 4× /26, 8× /27, or even smaller blocks depending on how many host bits you borrow. Each extra prefix bit halves the address space and reduces the usable host count. This is the basic tradeoff behind subnet planning: more subnets means fewer hosts per subnet.