Introduction
The CIDR Calculator helps you quickly work out network details from a CIDR notation address. CIDR stands for Classless Inter-Domain Routing.1 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. This calculator handles the subnet math so you can focus on building your network right.
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.1 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 came in three fixed sizes, called Class A, B, and C.1 A Class C network allowed at most 254 hosts, which was too small for many organizations, while a Class B network allowed up to 65,534, which was too large for most, so Class B addresses were running out.1 CIDR replaced the classes with blocks that can be any power-of-two size.1 It was first proposed in 1992 and set out in RFC 1519 in September 1993.1
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.1 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.1 Different prefix lengths also determine how a network is divided into smaller subnets.
Key Terms to Know
- Network Address: The first address in a CIDR block. It identifies the network itself.5
- Broadcast Address: The last address in a CIDR block. It is used to send data to all devices on the network.2
- Subnet Mask: A 32-bit number that separates the network part of an address from the host part.3 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).2
- Wildcard Mask: The inverse of the subnet mask.8 It is often used in router access control lists (ACLs).8
Why CIDR Matters
CIDR is important because it keeps the internet running efficiently. It slows the growth of internet routing tables and the rate at which IPv4 addresses are used up.1 Network administrators use CIDR every day to plan subnets, set up firewalls, and configure routers. Understanding CIDR notation is a core networking skill.