IP Subnet Calculator (CIDR)

What does it report?

For an IPv4 address and CIDR prefix: the network address, broadcast address, netmask in dotted and binary form, the usable host range, the host count, and the legacy address class.

Why is the usable host count two fewer than the total?

Because the first address in a block identifies the network itself and the last is the broadcast address — neither can be assigned to a machine. A /24 holds 256 addresses and 254 usable hosts.

Why doesn't that subtraction apply to /31 and /32?

Because those are special cases and the tool handles them. A /31 has no room for a network-and-broadcast split, and RFC 3021 defines it as a point-to-point link where both addresses are usable — that is how router interconnects are commonly numbered. A /32 is a single host route. Subtracting two from either would give a nonsensical zero or negative count.

What does the prefix length actually mean?

How many leading bits are fixed as the network portion. A /24 fixes the first 24 bits, leaving 8 for hosts; every bit you give back to the host portion doubles the block. That is why sizes go 256, 512, 1024 rather than in round decimal steps.

Why show the netmask in binary?

Because the binary form is where subnetting stops being memorisation. Seeing 255.255.255.0 as twenty-four ones followed by eight zeros makes it obvious why /25 splits a block in half and why the mask must be contiguous ones.

Are address classes still relevant?

Not operationally. Classful addressing was replaced by CIDR in the 1990s, and modern routing has no concept of class A, B or C. The class is reported because older documentation and interview questions still use the terms, not because anything routes by them.

Does it handle IPv6?

No — this is IPv4 only. IPv6 subnetting uses hexadecimal notation and a vastly larger space, where the host-count arithmetic here would be meaningless.

Why does it reject my address?

Each octet must be a decimal number from 0 to 255 with no leading text. Addresses with more or fewer than four octets, or an octet above 255, are rejected rather than silently wrapped.

Is my input transmitted?

No. The arithmetic is pure integer maths in your browser.