Subnet / CIDR calculator
Compute network address, broadcast/last address and host count for an IPv4 or IPv6 CIDR block. Pure arithmetic on the address you give it. No lookup, no network.
Runs on your device. The file is never uploaded.
Subnet / CIDR calculator expands one IPv4 or IPv6 block, say 192.168.1.0/24, into its network and last address, the hosts you can assign, and the total count. Big integers do the counting, so an IPv6 /32 reports an exact figure. A prefix outside the width of its family is refused, naming that width.
Questions
What does it give me for a block?
The address and prefix you entered with its family, the network address, the broadcast or last address, the usable host range with a host count for IPv4, and the total address count. Totals are computed as big integers, so a /8 or an IPv6 /32 prints an exact number rather than an approximation.
Does it handle IPv6?
Yes. Give it something like 2001:db8::/32 and you get the network and last address plus the exact total. The last address line notes that IPv6 has no broadcast concept, and there is no usable range line, because the first and last address reservations of IPv4 do not apply.
Why does a /31 show two usable addresses?
Because RFC 3021 says so. A /31 is a two address point to point link with no network or broadcast reservation, so both addresses are usable, and the tool says that on the usable line. A /32 is handled separately as a single host.
What error means my input was wrong?
If there is no slash it stops with a message asking for "address/prefix" and showing 192.168.1.0/24 as the example, with your input quoted back. A prefix outside the range for the family gives a message naming the prefix and the address width, such as a 32-bit address.
Does it look anything up on the network?
No. It is pure arithmetic on the address you typed: no whois, no DNS, no ping and no request of any kind. That also means it cannot tell you whether the block is allocated, routed or in use anywhere. It answers what the addresses are, not who has them.