🌐 CIDR Calculator

About the CIDR Calculator

Classless Inter-Domain Routing (CIDR) notation is the standard way to represent an IP address and its network prefix in a single expression: for example, 192.168.1.0/24. The number after the slash is the prefix length — it specifies how many bits of the address identify the network, with the remaining bits identifying individual hosts within that network.

Key terms explained

• Network address — the first address in the block, identifying the network itself. It cannot be assigned to a host (e.g., 192.168.1.0 in a /24).
• Broadcast address — the last address; packets sent here are delivered to all hosts on the network. Also not assignable to a host (e.g., 192.168.1.255).
• Subnet mask — the prefix expressed in dotted-decimal notation. A /24 prefix equals a mask of 255.255.255.0.
• Wildcard mask — the bitwise inverse of the subnet mask, used in Cisco ACLs and firewall rules (e.g., 0.0.0.255 for a /24).
• Usable hosts — the number of addresses available to assign to devices. For most prefixes this equals 2^(32−prefix) − 2 (subtracting the network and broadcast addresses).

Common use cases

• Planning VPC or VNet address spaces in AWS, Azure, or GCP
• Configuring firewall rules, security groups, and network ACLs
• Designing subnets for different application tiers (web, app, database)
• Verifying whether two IP addresses belong to the same subnet
• Understanding and troubleshooting IP routing tables

How to use this tool

Enter a CIDR block (e.g., 10.0.0.0/16) and click Calculate. The tool instantly shows the network address, broadcast address, subnet mask, wildcard mask, first and last usable host, and the total number of usable hosts. Click any of the prefix-length chips below the input to quickly switch to common prefix lengths without retyping the IP.