CIDR Calculator

What Is CIDR?

CIDR (Classless Inter-Domain Routing) is a way to write an IP address and its associated routing prefix in one compact form. Instead of specifying a subnet mask like 255.255.255.0, you write a slash followed by the number of bits used for the network part — for example, 192.168.1.0/24. The number after the slash is the prefix length: how many bits (from the left) define the network. The remaining bits define hosts within that network.

CIDR replaced the older classful addressing (Class A, B, C). It allows flexible subnet sizes and efficient allocation of IP space. When you design networks, set up firewalls, or configure cloud VPCs, you constantly work with CIDR blocks. Our subnet calculator and IP range calculator help you convert between CIDR, subnet mask, and actual IP ranges. For checking whether an address is valid before calculating, use our IP address validator.

How CIDR Notation Works

In 192.168.1.0/24, the /24 means the first 24 bits are fixed as the network identifier. An IPv4 address has 32 bits total, so 24 bits for network leaves 8 bits for hosts. Two to the power of 8 is 256 addresses; subtract the network and broadcast addresses and you get 254 usable host addresses. A /16 gives 65,536 addresses (65,534 usable); a /28 gives 16 addresses (14 usable). The smaller the number after the slash, the larger the block.

Converting from subnet mask to CIDR is simple: count the consecutive 1 bits in the mask. 255.255.255.0 in binary is 24 ones followed by 8 zeros, so the CIDR is /24. Our wildcard mask calculator inverts the subnet mask for ACLs; the network address calculator and broadcast address calculator give you the first and last addresses of the block.

Why Use a CIDR Calculator

Network engineers use CIDR calculators to plan subnets, check firewall rules, and understand cloud provider offerings (e.g. AWS VPC CIDR blocks). Developers use them to validate allowed IP ranges in configs or to split a large block into smaller subnets. Security teams use them to map which addresses belong to a given prefix. A CIDR calculator saves time and reduces errors compared to doing bitwise math by hand.

This calculator runs in your browser: enter any IPv4 address and a CIDR prefix (0–32), and you get the subnet mask, wildcard mask, network address, broadcast address, first and last host, and usable host count. For listing every IP in a range, use the IP range generator. For IPv6, use our IPv6 subnet calculator. To see if an IP is private or public, try the private vs public IP checker.

CIDR and Real-World Use

Cloud platforms (AWS, GCP, Azure) and on-prem networks rely on CIDR for defining subnets and security groups. Knowing the exact range of a /24 or /28 helps you write accurate firewall rules and avoid overlapping ranges when connecting networks. Pair a CIDR calculator with our IP lookup to see geolocation of an IP, or with ping test and traceroute to test connectivity to hosts in a subnet.

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