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.

What is the IPv4/IPv6 CIDR Range Calculator?

Compute the full details of any IPv4 or IPv6 CIDR block in your browser: network address, broadcast address, subnet mask, first and last usable host, wildcard mask and total host count. Built for firewall and network security engineers. It runs free in your browser on Gera Tools, with nothing uploaded.

IPv4/IPv6 CIDR Range Calculator

Name: IPv4/IPv6 CIDR Range Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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 in that block.

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 to 0.
network = address AND mask clears the host bits to give the base address.
broadcast = network OR (NOT mask) sets every host bit to 1, giving the last address in the range.
Total addresses = 2^(bits − prefix). For IPv4, usable hosts = total − 2 (network + broadcast), with /31 and /32 handled as special cases.

IPv6 has no broadcast; the tool reports first and last address without reserving one.

IPv4 example: 10.0.0.0/22

A /22 spans 1,024 addresses across four class-C blocks:

Field	Value
Subnet mask	255.255.252.0
Wildcard mask	0.0.3.255
Network address	10.0.0.0
Broadcast address	10.0.3.255
First usable host	10.0.0.1
Last usable host	10.0.3.254
Total addresses	1,024
Usable hosts	1,022

IPv6 example: 2001:db8::/48

An IPv6 /48 is a typical allocation to an organization. It contains 2^80 addresses — about 1.2 × 10^24. The tool shows:

Network: 2001:0db8:0000:0000:0000:0000:0000:0000
Last address: 2001:0db8:0000:ffff:ffff:ffff:ffff:ffff
Subnets of /64 available within it: 65,536

IPv4 vs IPv6: key differences in this calculator

Feature	IPv4	IPv6
Address size	32 bits	128 bits
Broadcast address	Yes (last address)	No (uses multicast instead)
Usable hosts	Total − 2	Total (no broadcast reservation)
Notation	Dotted decimal + prefix	Hex colon + prefix
Max prefix	/32 (single host)	/128 (single host)

Where CIDR ranges are used in practice

Cloud networking (AWS VPC, Azure VNet, GCP VPC): Each virtual network is defined by a CIDR block, and subnets are sub-ranges within it. The cloud provider reserves a few addresses for internal use, so the usable host count is slightly lower than this calculator shows for raw CIDR.

Firewall allow/block lists: CIDR notation is the standard way to permit a range of IPs in one rule rather than listing individual hosts.

BGP route advertisements: ISPs advertise their address space to the global routing table as CIDR prefixes.

Security group rules: Verifying that a /24 rule covers an address you want to allow (or inadvertently allows something you didn’t intend) is a common use of a CIDR calculator.

A /0 covers the entire address space and should only appear in a “allow all” or “default route” context. IPv6 ranges can be astronomically large, so the tool shows exact address counts as base-10 strings rather than rounding.