What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation expresses a network address together with its prefix length using a forward slash (e.g., 192.168.1.0/24). The number after the slash indicates how many of the 32 bits belong to the network portion. A larger prefix means a smaller subnet with fewer available host addresses. CIDR replaced the older Class A/B/C system because it allows far more flexible and efficient IP address allocation across the internet and within organisations.

What's the difference between subnet mask and wildcard mask?

A subnet mask marks network bits as 1 and host bits as 0 (e.g., 255.255.255.0 for /24). A wildcard mask is the binary complement — network bits are 0 and host bits are 1 (e.g., 0.0.0.255 for /24). While subnet masks define the network boundary on hosts and routers, wildcard masks are used in Cisco ACLs and OSPF configurations to specify which address ranges a rule applies to. Both carry the same information expressed differently.

Why are 2 addresses subtracted from usable hosts?

In every subnet, the first address (all host bits set to zero) is the network address, used to identify the subnet itself rather than any device. The last address (all host bits set to one) is the broadcast address, which delivers a packet simultaneously to every device in the subnet. Neither can be assigned to an individual host. For a /24 subnet there are 256 total addresses, but only 254 are usable for devices.

How do I split a network into smaller subnets?

Subnetting borrows bits from the host portion to create additional network segments. For example, splitting a /24 into /25 borrows one bit, giving two subnets (0–127 and 128–255) each with 126 usable hosts. Borrowing two bits creates four /26 subnets with 62 hosts each. Every additional borrowed bit doubles the number of subnets while halving the host count per subnet. Use this calculator to map out all the resulting ranges before making changes to a live network.

IP Subnet Calculator - Bunny Calculator

Complete User Guide

Our IP Subnet Calculator helps network administrators, students, and IT professionals determine subnet parameters from any IP address and prefix:

Step 1: Enter an IP address in dotted-decimal format (e.g., 192.168.1.100). All four octets must be between 0 and 255.

Step 2: Enter or select the CIDR prefix length (e.g., /24). This is the number of bits reserved for the network portion. Common values range from /8 (large enterprise networks) to /30 (point-to-point links).

Step 3: Click 'Calculate'. The tool immediately returns: - Network address (host bits set to zero) - Broadcast address (host bits set to one) - Subnet mask in dotted-decimal form - Wildcard mask (inverse of subnet mask, used in ACLs) - First and last usable host IP addresses - Total number of usable hosts - IP address class (A, B, C, or private range)

Step 4: Review the binary breakdown to see exactly which bits are network bits and which are host bits. This visual aid is especially useful when studying for CCNA, CompTIA Network+, or AWS certifications where subnetting questions appear on every exam.

About IP Subnet Calculator

IP subnetting divides a large network into smaller, isolated broadcast domains called subnets. Each subnet has its own IP address range and is separated from other subnets by a router, which improves performance and security.

Network engineers use subnetting daily when designing enterprise networks, configuring routers, writing access control lists, and troubleshooting connectivity issues. In cloud environments, every AWS VPC, Azure virtual network, and GCP subnet is defined using a CIDR block, making subnetting knowledge essential for cloud architects.

This calculator replaces the manual process of converting between binary and decimal — which is error-prone under exam pressure or time constraints. Use it to verify network designs, quickly look up host ranges during troubleshooting, or practise CIDR math before a certification exam like CCNA or CompTIA Network+.

Frequently Asked Questions

IP Subnet Calculator

What is CIDR notation?

What's the difference between subnet mask and wildcard mask?

Why are 2 addresses subtracted from usable hosts?

How do I split a network into smaller subnets?

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