How to Use the CIDR Calculator
The CIDR calculator is an essential tool for network administrators and engineers to quickly determine the parameters of a specific sub-network. By entering an IP address and a CIDR prefix (the "/n" notation), you can instantly visualize the logical boundaries of your network.
To get started, simply input your network or host IP and the desired prefix length. The tool will generate a comprehensive breakdown including the subnet mask, network ID, and usable host range.
- IP Address
- Enter the IPv4 address you wish to analyze (e.g., 192.168.1.15). This can be a specific host address or a network address.
- CIDR Prefix
- The number of bits used for the network portion of the address. This is a value between 0 and 32.
What is CIDR?
CIDR stands for Classless Inter-Domain Routing. Introduced in 1993, it replaced the older "Classful" network architecture (Class A, B, and C). CIDR allows for much more efficient allocation of IP addresses by using Variable Length Subnet Masking (VLSM). Instead of being locked into fixed 8, 16, or 24-bit masks, networkers can create subnets of any size required.
How It Works
When you calculate a CIDR block, the math relies on binary bit manipulation. An IPv4 address consists of 32 bits divided into four octets. The CIDR prefix indicates how many bits are "masked" for the network ID.
Subnet Mask = 232 – 2(32-Prefix)
- Network Address: Calculated by performing a bitwise AND operation between the IP and the Mask.
- Broadcast Address: Calculated by taking the Network Address and setting all host bits to 1.
- Usable Hosts: Calculated as 2(32-n) – 2 (subtracting the network and broadcast addresses).
Calculation Example
Example: You are assigned the IP 10.0.0.5 with a CIDR prefix of /27.
Step-by-step solution:
- Prefix /27: Means 27 bits for network, 5 bits for hosts.
- Subnet Mask: The binary is 11111111.11111111.11111111.11100000, which converts to 255.255.255.224.
- Network Address: 10.0.0.0 (The starting point of this 32-address block).
- Broadcast Address: 10.0.0.31 (The end point of the block).
- Usable Range: 10.0.0.1 to 10.0.0.30.
- Usable Hosts: 25 – 2 = 30.
CIDR Reference Table
| Prefix | Subnet Mask | Total Hosts |
|---|---|---|
| /32 | 255.255.255.255 | 1 |
| /30 | 255.255.255.252 | 4 |
| /28 | 255.255.255.240 | 16 |
| /24 | 255.255.255.0 | 256 |
| /20 | 255.255.240.0 | 4,096 |
| /16 | 255.255.0.0 | 65,536 |
| /8 | 255.0.0.0 | 16,777,216 |
Common Questions
Why are two hosts subtracted from the usable total?
In any standard subnet, the first address is reserved for the Network ID and the very last address is reserved for the Broadcast Address. Since these two addresses cannot be assigned to hardware devices like computers or printers, they are considered "unusable" for host assignment.
What is a /24 subnet?
A /24 subnet is one of the most common configurations in small business and home networking. It uses 24 bits for the network, leaving 8 bits for hosts. This results in a mask of 255.255.255.0 and allows for 254 usable host devices.
Can CIDR be used for IPv6?
Yes, CIDR notation is also the standard for IPv6. However, because IPv6 uses 128-bit addresses, the prefixes are much larger (standard LANs typically use a /64 prefix). This CIDR calculator is specifically designed for IPv4, which is still the dominant protocol for internal private networking.