A prefix length describes how much of an IPv4 address belongs to the network. The resulting block can include addresses reserved for conventional network and broadcast use, so total addresses and usable hosts are different figures.
Use IPv4 CIDR Subnet Calculator with a dotted IPv4 address and a prefix from 0 to 32. It calculates the block without performing a lookup or scan.
A common office example
For 192.168.1.10 with prefix 24, the network is 192.168.1.0/24. Its subnet mask is 255.255.255.0 and its total block contains 256 addresses.
Under conventional subnet use, the usable range is 192.168.1.1 through 192.168.1.254, giving 254 usable addresses. The last address is 192.168.1.255.
| Prefix | Address count behaviour |
|---|---|
| 24 | 256 total, 254 conventional usable |
| 31 | Two point to point endpoints |
| 32 | One address |
The /31 case follows the point to point exception rather than subtracting two from a block of two. Whether that layout fits your equipment still depends on its configuration and supported network design.
Arithmetic is not network discovery
The output does not tell you which address is assigned, whether a device responds or whether a route is reachable. No DNS lookup or packet scan occurs.
A calculated address can also be unsuitable for a particular application because of private, reserved or organisational policy rules. The subnet calculation does not check those deployment decisions.
This tool handles IPv4 only and requires 64 bit PHP for its arithmetic. It is not an IPv6 subnet planner.
When planning a network, keep the prefix, address range and intended use together. The figures help explain the block, but a working network also requires correct routing, interface settings and an allocation plan that avoids conflicts.
Reference: Point to point IPv4 prefix context.