CIDR Calculator

Calculate subnet masks, network addresses, broadcast addresses, and usable IP ranges from CIDR notation.

Convert CIDR notation to detailed subnet information including network address, broadcast address, subnet mask, and number of usable hosts for efficient network planning and administration.

Examples

Click on any example to load it into the calculator.

Home Network (/24)

Home Network

Typical home network configuration with 254 usable IP addresses.

IP Address: 192.168.1.0

CIDR Prefix: /24

Small Office (/26)

Small Office

Small office network with 62 usable IP addresses for departments.

IP Address: 10.0.1.0

CIDR Prefix: /26

Large Network (/16)

Large Network

Large corporate network with 65,534 usable IP addresses.

IP Address: 172.16.0.0

CIDR Prefix: /16

Point-to-Point (/30)

Point-to-Point

Point-to-point connection with 2 usable IP addresses.

IP Address: 10.0.0.0

CIDR Prefix: /30

Other Titles
Understanding CIDR Calculator: A Comprehensive Guide
Master the fundamentals of Classless Inter-Domain Routing (CIDR) and learn how to efficiently plan, design, and manage IP networks using modern subnetting techniques.

What is CIDR Calculator?

  • Core Concepts and Definitions
  • Why CIDR Matters
  • Evolution from Classful Addressing
The CIDR Calculator is an essential networking tool that converts Classless Inter-Domain Routing (CIDR) notation into detailed subnet information. CIDR represents a fundamental shift from the traditional classful IP addressing system to a more flexible, efficient method of allocating IP addresses. This calculator transforms simple CIDR notation (like 192.168.1.0/24) into comprehensive network details including subnet masks, network addresses, broadcast addresses, and usable IP ranges.
The Revolution of Classless Addressing
Before CIDR, IP addresses were divided into rigid classes (A, B, C) with fixed subnet masks, leading to inefficient address allocation and rapid IPv4 address exhaustion. CIDR introduced variable-length subnet masking (VLSM), allowing network administrators to create subnets of any size based on actual needs rather than predefined classes. This flexibility enables organizations to optimize their IP address usage, reduce waste, and implement more sophisticated network architectures.
Understanding CIDR Notation
CIDR notation combines an IP address with a prefix length, written as IPaddress/prefixlength. The prefix length indicates how many bits are used for the network portion of the address. For example, 192.168.1.0/24 means the first 24 bits represent the network, leaving 8 bits for host addresses. This creates a subnet with 256 total addresses (2^8), of which 254 are usable (excluding network and broadcast addresses).
Mathematical Foundation and Binary Logic
CIDR calculations rely on binary arithmetic and bitwise operations. The subnet mask is created by setting the first N bits to 1 (where N is the prefix length) and the remaining bits to 0. The network address is found by performing a bitwise AND operation between the IP address and subnet mask. The broadcast address is calculated by setting all host bits to 1. Understanding these binary operations is crucial for accurate CIDR calculations and network troubleshooting.

Key CIDR Concepts:

  • CIDR Notation: IP_address/prefix_length format (e.g., 192.168.1.0/24)
  • Subnet Mask: Binary mask showing network vs host bits (e.g., 255.255.255.0)
  • Network Address: First address in the subnet (all host bits = 0)
  • Broadcast Address: Last address in the subnet (all host bits = 1)

Step-by-Step Guide to Using the CIDR Calculator

  • Input Requirements and Validation
  • Calculation Process
  • Result Interpretation
Using the CIDR Calculator effectively requires understanding the input requirements, validation processes, and how to interpret the comprehensive results. This systematic approach ensures accurate network planning and prevents common subnetting errors that can lead to network connectivity issues.
1. Preparing Your Input Data
Start by identifying the IP address you want to subnet. This can be any valid IPv4 address in dotted decimal notation (e.g., 192.168.1.0, 10.0.0.0, 172.16.0.0). The IP address serves as the starting point for your subnet calculation. Next, determine the appropriate CIDR prefix length based on your network requirements. Consider factors like the number of hosts needed, network segmentation goals, and future growth plans when selecting the prefix length.
2. Understanding Prefix Length Selection
The CIDR prefix length directly determines your subnet size. Common prefix lengths include /24 (256 addresses, 254 usable), /25 (128 addresses, 126 usable), /26 (64 addresses, 62 usable), /27 (32 addresses, 30 usable), and /30 (4 addresses, 2 usable). For point-to-point links, /30 is typically sufficient. For small offices, /26 or /27 works well. For larger networks, /24 or /23 may be appropriate. Always plan for future growth when selecting prefix lengths.
3. Validating Input and Understanding Errors
The calculator validates both the IP address format and CIDR prefix range. IP addresses must follow the standard dotted decimal format with four octets between 0-255. CIDR prefixes must be integers between 0-32. Common validation errors include malformed IP addresses (missing dots, invalid octets), prefix lengths outside the valid range, and non-numeric prefix values. Understanding these validation rules helps prevent input errors.
4. Interpreting Comprehensive Results
The calculator provides seven key pieces of information: network address (the subnet's starting point), broadcast address (the subnet's ending point), subnet mask (binary mask in decimal format), total hosts (including network and broadcast), usable hosts (excluding network and broadcast), first usable IP (network address + 1), and last usable IP (broadcast address - 1). Each result serves a specific purpose in network configuration and documentation.

Common CIDR Prefix Lengths:

  • /24: 256 addresses, 254 usable (typical home/office network)
  • /25: 128 addresses, 126 usable (small office network)
  • /26: 64 addresses, 62 usable (department network)
  • /27: 32 addresses, 30 usable (small team network)
  • /30: 4 addresses, 2 usable (point-to-point connection)

Real-World Applications and Network Planning

  • Enterprise Network Design
  • Cloud Infrastructure Planning
  • Security and Segmentation
CIDR calculators are indispensable tools across various networking scenarios, from small office setups to large enterprise networks and cloud infrastructure. Understanding how to apply CIDR principles in real-world situations enables efficient network design, optimal resource utilization, and enhanced security.
Enterprise Network Architecture and Design
Large organizations use CIDR calculators to design hierarchical network architectures with multiple subnets for different departments, functions, and security zones. A typical enterprise might use /16 for the main network, /24 for department subnets, /26 for smaller teams, and /30 for point-to-point connections. This hierarchical approach enables efficient routing, simplified management, and enhanced security through network segmentation. CIDR planning also supports scalability, allowing organizations to add new subnets without disrupting existing infrastructure.
Cloud Infrastructure and Virtual Networks
Cloud providers and virtualization platforms heavily rely on CIDR calculations for virtual network design. AWS VPCs, Azure Virtual Networks, and Google Cloud VPCs all use CIDR notation for IP address allocation. Cloud architects use CIDR calculators to plan address spaces that accommodate current needs while allowing for future expansion. They must also consider overlapping address spaces when connecting on-premises networks to cloud environments, requiring careful CIDR planning to avoid conflicts.
Network Security and Segmentation
Security professionals use CIDR calculations to implement network segmentation strategies that isolate different security zones. DMZ networks, internal networks, guest networks, and IoT networks each require separate subnets with appropriate CIDR allocations. This segmentation enables granular access control, reduces attack surfaces, and simplifies security policy enforcement. CIDR planning also supports firewall rule creation, where specific subnets can be allowed or denied access based on their network addresses.

Network Design Best Practices:

  • Reserve address space for future growth (typically 20-30% extra)
  • Use consistent subnet sizes within similar functional areas
  • Document all CIDR allocations for troubleshooting and planning
  • Consider routing efficiency when designing hierarchical networks

Common Misconceptions and Troubleshooting

  • Address Space Planning Errors
  • Subnet Mask Confusion
  • Broadcast Address Misunderstanding
Even experienced network administrators can fall prey to common CIDR misconceptions that lead to network issues, inefficient designs, and troubleshooting challenges. Understanding these pitfalls and their solutions is crucial for successful network implementation.
Myth: All IP Addresses in a Subnet Are Usable
This misconception leads to incorrect host count planning and potential network capacity issues. Reality: Every subnet reserves two addresses—the network address (all host bits = 0) and the broadcast address (all host bits = 1). For example, a /24 subnet has 256 total addresses but only 254 usable addresses. This reservation is fundamental to IP networking and cannot be bypassed. Understanding this limitation is crucial for accurate capacity planning and avoiding network design errors.
Subnet Mask vs CIDR Prefix Confusion
Many people confuse subnet masks with CIDR prefixes, leading to configuration errors. A subnet mask (like 255.255.255.0) is the binary representation in decimal format, while a CIDR prefix (like /24) is the count of network bits. They represent the same information in different formats. The calculator converts between these formats automatically, but understanding the relationship helps with manual calculations and troubleshooting. For example, 255.255.255.0 equals /24 because there are 24 consecutive 1s in the binary mask.
Broadcast Address Misunderstanding
The broadcast address is often misunderstood as the last usable IP address, leading to configuration errors. The broadcast address is actually the last address in the subnet and cannot be assigned to hosts. It's used for network-wide communication where all hosts in the subnet receive the message. The last usable IP address is actually the broadcast address minus 1. This distinction is critical for DHCP server configuration, static IP assignment, and network documentation.

Troubleshooting Common Issues:

  • Host count mismatch: Always subtract 2 from total addresses for usable hosts
  • Subnet mask errors: Verify binary-to-decimal conversion accuracy
  • Broadcast confusion: Remember broadcast address is not usable for hosts
  • Address overlap: Ensure subnets don't overlap when planning multiple networks

Mathematical Derivation and Advanced Concepts

  • Binary Arithmetic and Bitwise Operations
  • Subnet Calculation Formulas
  • VLSM and Supernetting
Understanding the mathematical foundations of CIDR calculations enables network professionals to perform manual calculations, verify automated results, and develop deeper insights into network behavior. These mathematical concepts form the basis for advanced networking techniques and troubleshooting methodologies.
Binary Arithmetic and Subnet Mask Creation
Subnet masks are created using binary arithmetic where the first N bits (prefix length) are set to 1 and the remaining bits are set to 0. For example, a /24 prefix creates the binary mask 11111111.11111111.11111111.00000000, which converts to 255.255.255.0 in decimal. This binary mask is then used in bitwise AND operations with IP addresses to determine network addresses. Understanding this binary process enables manual verification of calculator results and deeper troubleshooting capabilities.
Network Address and Broadcast Address Calculations
The network address is calculated by performing a bitwise AND operation between the IP address and subnet mask. This operation preserves the network bits and sets all host bits to 0. The broadcast address is calculated by performing a bitwise OR operation between the network address and the inverse of the subnet mask, effectively setting all host bits to 1. These calculations are fundamental to understanding how IP addressing works and enable manual verification of automated results.
Variable Length Subnet Masking (VLSM) and Supernetting
VLSM allows different subnets within the same network to have different prefix lengths, optimizing address space usage. For example, a /24 network could be divided into a /25 (128 addresses) and two /26 subnets (64 addresses each). Supernetting combines multiple smaller networks into larger ones using shorter prefix lengths. For example, four /24 networks can be combined into a single /22 network. These advanced techniques require careful CIDR planning and calculation to avoid address conflicts and routing issues.

Advanced Calculation Examples:

  • VLSM: 192.168.1.0/24 divided into 192.168.1.0/25 and 192.168.1.128/26
  • Supernetting: 192.168.0.0/24 + 192.168.1.0/24 + 192.168.2.0/24 + 192.168.3.0/24 = 192.168.0.0/22
  • Binary conversion: /24 = 11111111.11111111.11111111.00000000 = 255.255.255.0