What does the prefix length mean?

The prefix length (the number after the slash) is how many leading bits of the address identify the network. A /24 fixes the first 24 bits, leaving 8 bits — 256 addresses — for hosts.

Why are usable hosts two fewer than the total for IPv4?

In a normal IPv4 subnet the first address is the network identifier and the last is the broadcast address, so neither is assignable to a host. A /24 has 256 total but 254 usable addresses.

Does IPv6 have a broadcast address?

No. IPv6 has no broadcast address; it uses multicast instead. This tool therefore shows the first and last address of an IPv6 range without reserving a broadcast, and does not subtract usable hosts.

How are /31 and /32 handled?

A /32 is a single host with no broadcast. A /31 is a two-address point-to-point link (RFC 3021) where both addresses are usable. The tool reports these special cases instead of subtracting two.

Is my input sent to a server?

No. All bit arithmetic is performed locally in JavaScript using BigInt, so the addresses you enter never leave your browser.

IPv4/IPv6 CIDR Range Calculator

A CIDR block (Classless Inter-Domain Routing) describes a contiguous range of IP addresses using an address and a prefix length, such as 192.168.1.0/24. This calculator expands any IPv4 or IPv6 CIDR into its constituent parts — network address, mask, host range and address count — entirely in your browser. It is built for firewall rule authors, cloud network engineers and anyone verifying an allow or block list.

How it works

The prefix length tells you how many leading bits are fixed as the network portion. The remaining bits vary across hosts.

The address is parsed into a single integer (32-bit for IPv4, 128-bit for IPv6 using BigInt).
A network mask is built with the top prefix bits set to 1 and the rest 0.
network = address AND mask clears the host bits to give the network (base) address.
broadcast = network OR (NOT mask) sets every host bit to give the last address in the range.
Total addresses = 2^(bits − prefix). For IPv4, usable hosts = total − 2 (network + broadcast), except /31 (2 usable, RFC 3021) and /32 (1 host).

IPv6 has no broadcast address, so the tool reports first and last addresses without reserving one.

Example

For 10.0.0.0/22:

Mask 255.255.252.0, wildcard 0.0.3.0
Network 10.0.0.0, broadcast 10.0.3.255
First usable 10.0.0.1, last usable 10.0.3.254
Total 1024 addresses, 1022 usable

Notes

A /0 covers the entire address space and a /32 (or IPv6 /128) is a single host. IPv6 ranges can be astronomically large, so the tool shows the exact address count as a base-10 string rather than rounding.