What is CIDR Notation?
CIDR (Classless Inter-Domain Routing) notation is a compact method for specifying IP address ranges and network masks. Introduced in 1993 to replace the older classful network addressing system, CIDR notation combines an IP address with a suffix indicating the number of bits in the network portion of the address. For example, 192.168.1.0/24 indicates that the first 24 bits define the network, leaving 8 bits for host addresses.
The CIDR notation revolutionized IP address allocation by allowing more flexible division of IP address space compared to the rigid Class A, B, and C boundaries. This flexibility is essential in modern networking where organizations need varying numbers of IP addresses. Our CIDR calculator helps you quickly convert between CIDR notation and traditional subnet masks, and calculate all relevant network details. For more comprehensive subnet calculations, you can also use our main subnet calculator tool.
Quick Tip
The number after the slash in CIDR notation directly tells you how many bits are used for the network portion. A /24 network uses 24 bits for the network (leaving 8 bits = 256 addresses), while a /16 uses 16 bits (leaving 16 bits = 65,536 addresses).
How to Use the CIDR Calculator
Our CIDR calculator offers two modes of operation to handle different scenarios you might encounter:
Mode 1: CIDR to Network Details
Enter a complete CIDR notation (like 10.0.0.0/8 or 172.16.0.0/12) and the calculator will provide comprehensive network information including the subnet mask, wildcard mask, network boundaries, and the full IP range. This mode is perfect when you have a CIDR block and need to understand its scope for firewall rules or network planning.
Mode 2: Subnet Mask to CIDR
If you have a traditional subnet mask and need to convert it to CIDR notation, use the second tab. Simply enter the subnet mask (like 255.255.255.0) and get the equivalent CIDR prefix. This is particularly useful when working with older network documentation or devices that display subnet masks in dotted-decimal format. For more detailed mask conversions, check out our dedicated netmask to CIDR converter.
Understanding CIDR Calculator Results
Network and Broadcast Addresses
Every CIDR block has two special addresses that cannot be assigned to hosts. The network address is the first IP in the range and identifies the network itself. The broadcast address is the last IP and is used to send data to all hosts in the network simultaneously. For example, in 192.168.1.0/24, the network address is 192.168.1.0 and the broadcast address is 192.168.1.255. You can calculate these individually using our network address calculator or broadcast address calculator.
Wildcard Mask
The wildcard mask is the bitwise inverse of the subnet mask. While subnet masks use 1s to identify network bits, wildcard masks use 0s. This inverse relationship makes wildcard masks essential for configuring access control lists (ACLs) on Cisco routers and OSPF routing configurations. A /24 subnet mask of 255.255.255.0 has a wildcard mask of 0.0.0.255. Our wildcard mask calculator provides additional functionality for these calculations.
Usable Host Range
The usable host range excludes the network and broadcast addresses, giving you the actual IP addresses that can be assigned to devices. For a /24 network with 256 total addresses, you have 254 usable host addresses (256 - 2 = 254). Understanding this range is crucial when planning IP address allocation for your network segments.
Common CIDR Blocks Reference
Here's a reference table showing commonly used CIDR blocks and their characteristics:
| CIDR | Subnet Mask | Total IPs | Usable Hosts | Typical Use |
|---|---|---|---|---|
| /32 | 255.255.255.255 | 1 | 1 | Single host route |
| /31 | 255.255.255.254 | 2 | 2 | Point-to-point links |
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point links |
| /29 | 255.255.255.248 | 8 | 6 | Small networks |
| /28 | 255.255.255.240 | 16 | 14 | Small office |
| /27 | 255.255.255.224 | 32 | 30 | Department |
| /26 | 255.255.255.192 | 64 | 62 | Medium office |
| /25 | 255.255.255.128 | 128 | 126 | Branch office |
| /24 | 255.255.255.0 | 256 | 254 | Standard LAN |
| /23 | 255.255.254.0 | 512 | 510 | Large office |
| /22 | 255.255.252.0 | 1,024 | 1,022 | Campus network |
| /16 | 255.255.0.0 | 65,536 | 65,534 | Enterprise |
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 | Major ISP |
CIDR and Subnetting
CIDR notation is fundamentally connected to subnetting. When you subnet a network, you're essentially increasing the CIDR prefix length to create smaller networks. For instance, a /24 network can be divided into two /25 networks, four /26 networks, or eight /27 networks. This hierarchical division allows network administrators to efficiently allocate IP addresses based on the actual requirements of each network segment.
Understanding CIDR is essential when working with Variable Length Subnet Masking (VLSM), which allows different subnets within the same network to have different sizes. This technique maximizes IP address utilization by assigning appropriately sized subnets to each network segment. For complex VLSM calculations, our VLSM calculator can help you design efficient address allocation schemes.
CIDR in Cloud Computing
CIDR notation is extensively used in cloud computing platforms like AWS, Azure, and Google Cloud. When creating Virtual Private Clouds (VPCs) or virtual networks, you specify the address range using CIDR blocks. Understanding CIDR is crucial for:
- Defining VPC address ranges (commonly /16 for main VPC)
- Creating subnets within VPCs (typically /24 for individual subnets)
- Configuring security group rules with IP ranges
- Setting up VPC peering with non-overlapping CIDR blocks
- Planning hybrid cloud connectivity with on-premises networks
When planning cloud infrastructure, it's essential to choose CIDR blocks that don't overlap with your existing networks. Our CIDR to IP range calculator can help you visualize the exact addresses in each block to ensure no conflicts exist.
CIDR Aggregation and Supernetting
CIDR also enables route aggregation (supernetting), where multiple smaller networks can be advertised as a single larger network. For example, four consecutive /26 networks can be aggregated into a single /24 route. This reduces the size of routing tables and improves routing efficiency across the internet. Route aggregation is a key technique used by ISPs and large enterprises to maintain manageable routing tables.
Aggregation Example
These four /26 networks:
192.168.1.0/26(192.168.1.0 - 192.168.1.63)192.168.1.64/26(192.168.1.64 - 192.168.1.127)192.168.1.128/26(192.168.1.128 - 192.168.1.191)192.168.1.192/26(192.168.1.192 - 192.168.1.255)
Can be aggregated into: 192.168.1.0/24
Private CIDR Blocks
RFC 1918 defines three private IP address ranges that are commonly expressed in CIDR notation. These addresses are not routable on the public internet and are used for internal networks:
- 10.0.0.0/8 - Class A private range (16,777,216 addresses)
- 172.16.0.0/12 - Class B private range (1,048,576 addresses)
- 192.168.0.0/16 - Class C private range (65,536 addresses)
Most home and office networks use addresses from the 192.168.0.0/16 range, while larger enterprises often use the 10.0.0.0/8 range for its larger address space. To check if an IP address falls within these private ranges, use our IP address validator tool.