How to use the subnet calculator
- Type a CIDR block into the field, such as 192.168.1.0/24 or 10.0.0.0/16.
- Press "Calculate".
- Read the network address, broadcast address, subnet mask and first and last usable hosts.
- Use the total and usable host counts to plan how many devices a subnet can hold.
What each result means
| Value | Meaning |
|---|---|
| Network address | The first address of the subnet, which identifies the subnet itself. |
| Broadcast address | The last address, used to send a message to every device in the subnet at once. |
| Subnet mask | The dotted mask that marks which part of the address is the network and which part is the host. |
| First / last usable host | The range of addresses you can actually assign to devices. |
| Total hosts | Every address in the subnet, including the network and broadcast addresses. |
| Usable hosts | The addresses available for real devices, excluding the two reserved addresses. |
How CIDR notation works
CIDR (Classless Inter-Domain Routing) notation packs an address and its prefix length into one value separated by a slash. The number after the slash says how many leading bits belong to the network. A /24 prefix leaves 8 bits for hosts, which is 256 addresses in total and 254 that are usable. A /16 leaves 16 bits for hosts — 65,536 addresses — and so on. The prefix is a compact way to describe a subnet without spelling out the full mask.
Why subnet math matters
Subnetting is how one network is divided into smaller, manageable pieces. Network engineers use it to keep departments or services isolated, to limit broadcast traffic, and to conserve address space. Getting the math right means two devices you intend to talk to each other actually land in the same subnet, while devices you want separate stay apart. A calculator like this removes the manual binary work and the mistakes that come with it.
A note on special cases
Most subnets reserve two addresses — the network address and the broadcast address — which is why the usable count is two less than the total. Very small prefixes are the exception: a /31 is used for point-to-point links where both addresses are usable, and a /32 describes a single host. The calculator applies these rules automatically so the numbers you see match how real routing equipment behaves.