What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation expresses an IP address and its associated network prefix length as a single string, e.g. 192.168.1.0/24. The number after the slash indicates how many leading bits are fixed (the network part); the remaining bits identify individual hosts.

How many usable hosts does a /24 subnet have?

A /24 subnet has 2^8 = 256 total addresses. Two are reserved — the network address and the broadcast address — leaving 254 usable host addresses.

What are /31 and /32 subnets used for?

A /32 identifies a single host (no network or broadcast). A /31 is used for point-to-point links per RFC 3021; both addresses are usable as host addresses, giving 2 hosts with no broadcast.

What is a wildcard mask?

A wildcard mask is the bitwise inverse of the subnet mask. It is used in ACL rules on Cisco IOS and in OSPF area statements to specify which bits of an address must match.

Does this tool support IPv6?

No. This tool handles IPv4 CIDR notation only (prefix lengths /0 through /32). IPv6 subnetting works differently and requires a separate tool.

What happens if I enter a host address instead of a network address?

The tool accepts any IPv4 address. It applies the subnet mask to derive the actual network address, so entering 192.168.1.50/24 gives the same network address (192.168.1.0) as entering 192.168.1.0/24.

What IP range is 192.168.1.0/24?

192.168.1.0 to 192.168.1.255. The network address is 192.168.1.0, the broadcast address is 192.168.1.255, and the usable hosts are 192.168.1.1 to 192.168.1.254.

Why does my cloud subnet have fewer usable IPs?

AWS and Azure reserve 5 addresses in every subnet: the first four and the last one. A /24 there leaves 251 usable addresses instead of 254, and a /28 leaves 11 instead of 14.

CIDR Calculator

Enter a CIDR block (e.g. 10.0.0.0/8) to instantly see network address, broadcast, subnet mask, usable host range, and total hosts.

A /n prefix fixes the first n of the 32 address bits.

CIDR Block

IPv4 Address / Prefix

Quick examples

Prefix / Host split

24 bits

8 bits

Network Host

Subnet Details

IP Address —

Network Address —

Broadcast —

Subnet Mask —

Wildcard Mask —

First Host —

Last Host —

Total Addresses —

Usable Hosts —

Binary Representation

IP Address —

Subnet Mask —

Network —

Copied!

Quick answer

A /n prefix fixes the first n of the 32 address bits. The block holds 2^(32 − n) addresses; subtract 2 for the network and broadcast addresses to get usable hosts (/31 and /32 are the exceptions).

Examples

CIDR	Subnet mask	Usable hosts
/8	255.0.0.0	16,777,214
/16	255.255.0.0	65,534
/22	255.255.252.0	1,022
/24	255.255.255.0	254
/27	255.255.255.224	30
/28	255.255.255.240	14
/30	255.255.255.252	2

Enter a CIDR block.

Summary

This CIDR calculator takes any IPv4 address in CIDR notation (e.g. 192.168.1.0/24) and breaks it down into all key subnet details: network address, broadcast address, subnet mask, wildcard mask, first and last usable host, total hosts, and usable hosts. Results update instantly in the browser without any server requests.

How it works

Enter an IPv4 address with a prefix length such as 192.168.1.0/24 in the input field.
The tool splits the notation into the host IP and prefix length (0–32).
It computes the subnet mask by setting the top N bits of a 32-bit integer.
Network address = IP AND mask; Broadcast = IP OR (NOT mask).
First usable host = Network + 1; Last usable host = Broadcast - 1.
Usable host count = 2^(32 - prefix) - 2 (or 2 for /31, 1 for /32).

Use cases

Plan IP address allocation for VPCs, data centers, or home networks.
Verify subnet boundaries before configuring routers or firewalls.
Calculate the broadcast address needed for ACL or routing rules.
Determine the number of usable hosts in a proposed subnet.
Understand supernetting by testing large prefix lengths like /8 or /16.
Cross-check network documentation and IP scheme designs.
Teach or learn subnetting concepts interactively.
Validate CIDR blocks in Terraform, Ansible, or cloud configs.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu