Subnet Calculator (IPv4 CIDR)
Full IPv4 subnet breakdown: network, broadcast, mask, wildcard, host range and count.
Results update as you type.
About this calculator
This subnet calculator turns an IPv4 address and a CIDR prefix into the full structure of a subnet. An IPv4 address is 32 bits, usually written as four dot-separated octets like 192.168.1.10, and a CIDR prefix such as /24 splits those 32 bits into a network part (the leading bits, fixed for every host in the subnet) and a host part (the remaining bits, unique to each device). This calculator takes an address and a prefix length and derives the network address, broadcast address, subnet mask, wildcard mask and the range of usable host addresses, along with the IP class and total address count.
The size of a subnet is set by the prefix: a /n block holds 2^(32−n) addresses, of which two are reserved — the network address (all host bits 0, naming the subnet) and the broadcast address (all host bits 1, reaching every host). So a /24 has 2^8 = 256 addresses and 254 usable hosts, a /25 has 128 addresses and 126 usable hosts, and each extra prefix bit halves the block. As a worked example, 192.168.1.10/24 gives a network of 192.168.1.0, a broadcast of 192.168.1.255, a 255.255.255.0 mask, a 0.0.0.255 wildcard mask, and a usable host range of 192.168.1.1 to 192.168.1.254.
Subnetting is how networks are carved into manageable, secure segments — separating departments, sizing VLANs, planning address allocation and writing firewall and routing rules. The wildcard mask (the bitwise inverse of the subnet mask) is what Cisco access-control lists and OSPF use to match ranges of addresses. The /31 and /32 prefixes are special cases: /31 is used for point-to-point links with two usable addresses and no broadcast, and /32 describes a single host.
Frequently asked questions
How many hosts are in a subnet?
A /n subnet has 2^(32−n) addresses, minus two reserved for the network and broadcast. A /24 has 256 addresses and 254 usable hosts.
What is a wildcard mask?
A wildcard mask is the bitwise inverse of the subnet mask (255.255.255.0 → 0.0.0.255). Access-control lists and OSPF use it to match ranges of addresses.
What is the difference between network and broadcast address?
The network address (all host bits 0) names the subnet; the broadcast address (all host bits 1) reaches every host on it. Neither can be assigned to a device.
What does the CIDR prefix (/24, /16) mean?
It is the number of leading bits that form the network portion of the address. A larger prefix means more network bits and fewer host bits, so a smaller subnet: /24 gives 254 hosts, /25 gives 126, and each extra bit halves the block.
How do I split a network into smaller subnets?
Borrow host bits by increasing the prefix. Splitting a /24 into two /25s gives two blocks of 126 hosts; going to /26 gives four blocks of 62. Each added prefix bit doubles the number of subnets and halves their size.
What is an IP address class?
The old classful scheme split addresses by their first octet: Class A (1–126), Class B (128–191) and Class C (192–223) for unicast, plus D for multicast and E reserved. CIDR replaced classes for allocation, but the calculator still reports the class the address falls in.
What are /31 and /32 subnets used for?
A /31 has just two addresses and no broadcast, used for point-to-point links between routers. A /32 is a single host address, common for loopbacks and host routes. The calculator handles both special cases.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/subnet/curl
curl "https://calculator.free/api/v1/subnet/?ip=192.168.1.10&cidr=24"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/subnet/?" + new URLSearchParams({
"ip": "192.168.1.10",
"cidr": "24"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.