Calculate CIDR notation subnets, IP ranges, available hosts, wildcard masks, and network addresses. Essential networking tool for IP address planning.
The CIDR Calculator turns a CIDR block into the subnet details you normally have to work out by hand: network address, broadcast address, usable host range, subnet mask, wildcard mask, and total host count. It is designed for the day-to-day subnetting tasks that come up in routing, cloud planning, labs, and certification study. That makes it useful when you want the network math, host range, and mask details in one place before you apply them to a real environment.
Enter an IP address and prefix length, such as 192.168.1.0/24, to see the resulting network breakdown. The tool shows where the subnet starts and ends, how many usable hosts it provides, and what the corresponding dotted-decimal mask looks like.
It also supports subnet splitting and supernetting, which makes it easier to reason about larger address plans and carve a network into smaller blocks. The prefix reference table gives you a quick way to compare common CIDR sizes, host counts, and subnet masks without recalculating everything each time.
Use this calculator to size subnets before you create VLANs, VPCs, ACLs, firewall rules, or lab environments. It keeps the host range, subnet mask, wildcard mask, and prefix length in one place, which reduces mistakes when you are planning or documenting an address layout. It is especially handy when you need to cross-check both host capacity and exact address boundaries quickly.
Network Address = IP AND Subnet Mask. Broadcast = Network OR Wildcard Mask. Host Count = 2^(32 - prefix) - 2. Subnet Mask = 32 bits with 'prefix' leading 1s. Wildcard Mask = NOT(Subnet Mask). First Host = Network + 1. Last Host = Broadcast - 1.
Result: Network: 192.168.1.0/24, Hosts: 254
A /24 network has a subnet mask of 255.255.255.0, giving 256 addresses (192.168.1.0 to 192.168.1.255). Subtracting network and broadcast addresses leaves 254 usable host addresses.
CIDR was introduced in 1993 to replace the wasteful Class A/B/C IP address system. Instead of fixed 8/16/24-bit network portions, CIDR allows any prefix length from /0 to /32, enabling precise allocation of address space. This dramatically slowed the exhaustion of IPv4 addresses and is fundamental to modern networking.
Subnetting divides a network into smaller, more manageable pieces. A company with a /24 might create separate subnets for different departments: /26 for engineering (62 hosts), /27 for sales (30 hosts), and /28 for management (14 hosts). This improves security, reduces broadcast traffic, and makes network management easier.
Cloud platforms like AWS, Azure, and GCP use CIDR extensively. When creating a VPC, you specify a CIDR block (typically /16 to /24). Within the VPC, subnets are created with smaller CIDR blocks. Understanding CIDR is essential for properly sizing networks, configuring security groups, and establishing VPN connections between cloud and on-premises networks.
The IPv4 address space (2^32 = 4.3 billion addresses) is effectively exhausted. IPv6 with its 128-bit addresses provides a virtually unlimited supply. However, IPv4 remains dominant, and efficient subnetting through CIDR remains important for managing existing allocations.
CIDR notation combines an IP address with a prefix length, such as 192.168.1.0/24. The prefix length tells you how many bits belong to the network portion, so the rest are available for hosts. That is why CIDR is so useful for both fixed-size and variable-size subnet plans.
Hosts = 2^(32 - prefix) - 2. The subtraction of 2 accounts for the network address and broadcast address, which cannot be assigned to hosts. For example, a /24 has 2^8 - 2 = 254 usable host addresses, while a /30 has only 2 usable addresses.
A wildcard mask is the inverse of a subnet mask, and it is commonly used in ACLs and OSPF network statements. Where the subnet mask shows which bits are fixed, the wildcard mask shows which bits may vary. For 255.255.255.0, the wildcard mask is 0.0.0.255.
They describe the same subnet size in two different formats. /24 is CIDR notation, which says 24 bits are reserved for the network, while 255.255.255.0 is the dotted-decimal subnet mask with 24 leading 1s. CIDR is shorter and easier to aggregate, while dotted-decimal masks are still common in older documentation and device interfaces.
10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are the RFC 1918 private ranges. They are reserved for internal networks and are not routable on the public internet. That makes them the standard choice for offices, home networks, labs, and cloud VPCs.
A /24 can be split into 2× /25, 4× /26, 8× /27, or even smaller blocks depending on how many host bits you borrow. Each extra prefix bit halves the address space and reduces the usable host count. This is the basic tradeoff behind subnet planning: more subnets means fewer hosts per subnet.