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The Locolory toolkit / Developer & Systems

CIDR IPv4 Subnet & Usable IP Calculator

Calculate network address, broadcast IP, netmask, wildcard mask, and usable host ranges.

What is CIDR notation? Classless Inter-Domain Routing combines an IP address with a prefix length, such as /24, indicating how many leading bits identify the network.

  • Key formula: Total IPv4 addresses = 2^(32 − prefix); the prefix creates the netmask, and the inverted netmask creates the wildcard and host portion.

All IPv4 arithmetic runs locally. Verify cloud-provider reservations and routing requirements before provisioning infrastructure.

● Calculated 100% locally in browser

IPv4 address + CIDR prefix

Reservation mode

Netmask

255.255.255.0

Wildcard mask

0.0.0.255

Network IP

192.168.1.0

Broadcast IP

192.168.1.255

First usable

192.168.1.1

Last usable

192.168.1.254

Total addresses

256

Usable addresses

254

Input IP in 32-bit binary

11000000101010000000000100000000

Conventional, /31, and /32 host rules

Conventional subnets reserve network and broadcast addresses. RFC 3021 allows both addresses of a /31 on a point-to-point link, while a /32 identifies a single address.

Whether a platform accepts those prefixes depends on the interface, routing system, and provider.

AWS VPC reservation model

AWS documents five unavailable IPv4 addresses per ordinary subnet: the first four and the last. Its supported IPv4 VPC subnet sizes currently range from /16 to /28.

The AWS mode highlights prefixes outside that range rather than implying they can be provisioned.

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Good questions. Clear answers.

What does a /24 CIDR prefix mean?

A /24 fixes the first 24 bits as the network portion and leaves 8 host bits. It contains 256 total IPv4 addresses, conventionally 254 usable hosts, and uses netmask 255.255.255.0.

Why are network and broadcast addresses normally unavailable to hosts?

In a conventional IPv4 subnet, the all-zero host portion identifies the network and the all-one host portion is the directed broadcast address. RFC 3021 provides a specific /31 exception for point-to-point links.

How many IPv4 addresses does AWS reserve in a VPC subnet?

AWS currently reserves the first four and final IPv4 address in each ordinary VPC subnet. AWS permits IPv4 subnet prefixes from /16 through /28; BYOIP behavior can differ.