Introduction
The CIDR Calculator helps you quickly work out network details from a CIDR notation address. CIDR stands for Classless Inter-Domain Routing. It is a way to describe a range of IP addresses using a single address and a prefix length, like 192.168.1.0/24. This tool takes that input and gives you useful information such as the network address, broadcast address, subnet mask, number of usable hosts, and the full range of IP addresses in the block. Whether you are setting up a home network, planning subnets for a business, or studying for a networking exam, this calculator makes the math easy so you can focus on building your network right. For more detailed subnet breakdowns, you can also use our Subnet Calculator.
How to Use Our CIDR Calculator
Enter an IP address and a CIDR prefix length to calculate your network address, broadcast address, subnet mask, total hosts, and usable host range.
IP Address: Type in the IP address you want to analyze. This is the base address of the network or any host address within it. It should be in standard IPv4 format, such as 192.168.1.0.
CIDR Prefix Length: Enter the prefix length, which is the number after the slash (/) in CIDR notation. This value ranges from 0 to 32 and tells the calculator how many bits are used for the network portion of the address. For example, a /24 prefix means the first 24 bits define the network, leaving 8 bits for host addresses.
Understanding CIDR Notation
CIDR stands for Classless Inter-Domain Routing. It is a method used to assign and organize IP addresses on a network. Before CIDR, networks were split into fixed classes (Class A, B, and C), which wasted a lot of IP addresses. CIDR was created in 1993 to solve this problem by allowing more flexible address allocation.
How CIDR Works
A CIDR address looks like a normal IP address with a slash and a number at the end. For example: 192.168.1.0/24. The number after the slash is called the prefix length. It tells you how many bits of the address are used for the network part. The remaining bits are used for individual devices (called hosts) on that network.
The prefix length can range from /0 to /32 for IPv4 addresses. A smaller prefix number means a bigger network with more available host addresses. A larger prefix number means a smaller network with fewer hosts. For example, a /24 network has 256 total addresses (254 usable for hosts), while a /16 network has 65,536 total addresses. Our Subnet Calculator can help you explore how different prefix lengths divide a network into smaller subnets.
Key Terms to Know
- Network Address: The first address in a CIDR block. It identifies the network itself.
- Broadcast Address: The last address in a CIDR block. It is used to send data to all devices on the network.
- Subnet Mask: A 32-bit number that separates the network part of an address from the host part. For a /24 network, the subnet mask is 255.255.255.0.
- Usable Hosts: The total number of addresses minus two (the network address and the broadcast address).
- Wildcard Mask: The inverse of the subnet mask. It is often used in router access control lists (ACLs).
Why CIDR Matters
CIDR is important because it keeps the internet running efficiently. It reduces wasted IP addresses and makes routing tables smaller, which helps routers work faster. Network administrators use CIDR every day to plan subnets, set up firewalls, and configure routers. Whether you are setting up a home network or managing a large data center, understanding CIDR notation is a core networking skill.
Frequently Asked Questions
What is a CIDR calculator?
A CIDR calculator is a tool that takes an IP address and a prefix length (like 192.168.1.0/24) and figures out the network details for you. It tells you the network address, broadcast address, subnet mask, wildcard mask, total addresses, usable hosts, and the range of usable IP addresses in that block.
What does the /24 mean in an IP address like 192.168.1.0/24?
The /24 is the prefix length. It means the first 24 bits of the 32-bit IP address are used for the network part. The remaining 8 bits are for host devices. A /24 gives you 256 total addresses and 254 usable host addresses.
What is the difference between total addresses and usable hosts?
Total addresses is every IP in the block. Usable hosts is that number minus two. The first address is the network address and the last address is the broadcast address. Neither can be assigned to a device. For example, a /24 block has 256 total addresses but only 254 usable hosts.
What is a wildcard mask and when do I need it?
A wildcard mask is the opposite of a subnet mask. Where the subnet mask has a 1, the wildcard mask has a 0, and vice versa. For a /24 network, the subnet mask is 255.255.255.0 and the wildcard mask is 0.0.0.255. Wildcard masks are used in router configurations, especially in access control lists (ACLs) on Cisco routers.
What is the difference between a network address and a broadcast address?
The network address is the first IP in a CIDR block. It identifies the network itself and cannot be assigned to a device. The broadcast address is the last IP in the block. It is used to send data to every device on that network at once. Neither address can be used as a regular host address.
Can I enter just an IP address and a subnet mask instead of CIDR notation?
Yes. This calculator has two input modes. You can switch to "IP Address + Subnet Mask" mode using the toggle at the top. Then type your IP address and pick a subnet mask from the dropdown. The calculator will give you the same results as if you entered CIDR notation.
What are IP classes A, B, C, D, and E?
IP classes are the old way of grouping IP addresses before CIDR was created:
- Class A: 0.0.0.0 to 127.255.255.255 — very large networks
- Class B: 128.0.0.0 to 191.255.255.255 — medium networks
- Class C: 192.0.0.0 to 223.255.255.255 — small networks
- Class D: 224.0.0.0 to 239.255.255.255 — multicast
- Class E: 240.0.0.0 to 255.255.255.255 — reserved
This calculator shows the IP class for your address, but CIDR has mostly replaced this system.
What does Private (RFC 1918) mean in the IP Type result?
RFC 1918 defines three ranges of IP addresses set aside for private networks. These addresses are not routed on the public internet. They are:
- 10.0.0.0 to 10.255.255.255
- 172.16.0.0 to 172.31.255.255
- 192.168.0.0 to 192.168.255.255
If your IP falls in one of these ranges, the calculator labels it as Private (RFC 1918).
What is a /31 or /32 network used for?
A /32 network has just one address. It is used to identify a single host, often in routing tables or firewall rules. A /31 network has two addresses and is used for point-to-point links between two routers. Since there are only two devices, no network or broadcast address is needed, so both addresses are usable.
What do the colored binary bits mean in the results?
The binary section uses two colors to show which bits belong to the network part and which belong to the host part. Blue bits are the network bits (set by the prefix length). Red bits are the host bits. This helps you see exactly where the network and host portions divide in the address.
How do I figure out how many subnets I can create from a larger network?
Divide the larger network into smaller ones by increasing the prefix length. Each extra bit in the prefix doubles the number of subnets. For example, splitting a /24 into /26 subnets gives you 4 subnets (2 extra bits = 2² = 4), each with 64 total addresses and 62 usable hosts.
What is the Integer / Hex ID in the results?
The Integer ID is your IP address written as a single whole number instead of four octets. The Hex ID is the same value in hexadecimal (base-16) format. These are useful for programmers and low-level network tools that store IP addresses as numbers rather than dotted-decimal strings.
Does this calculator work with IPv6 addresses?
No. This calculator only works with IPv4 addresses (like 192.168.1.0/24). IPv6 addresses use a different format and much longer address space. You would need a separate IPv6 CIDR calculator for those.
What is a common CIDR block for a home network?
Most home routers use 192.168.1.0/24 or 192.168.0.0/24. This gives you 254 usable host addresses, which is more than enough for home devices like phones, computers, smart TVs, and other gadgets.
Can I copy the results from the calculator?
Yes. Each result row has a Copy button next to it. Click it to copy that value to your clipboard. You will see a brief "Copied" message to confirm it worked.