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Guide

CIDR Notation Explained

CIDR notation describes an IP network by combining a starting address with a prefix length, such as 8.8.8.0/24.

Open the CIDR calculator

The slash number

The number after the slash is the prefix length. It says how many leading bits belong to the network part of the address.

A larger prefix number means a smaller network. For IPv4, /24 contains 256 addresses while /32 is one address.

Host-prefix input

People often type a host address with a network prefix, for example 8.8.8.8/24. The canonical network is 8.8.8.0/24 because the host bits are cleared.

ShowIP accepts host-prefix input and normalizes it before calculating first address, last address, netmask, broadcast, and total address count.

Key terms

CIDR
Classless Inter-Domain Routing
Prefix length
Number of fixed leading network bits
Host bits
Remaining address bits inside the network

Examples

CIDR calculator Normalize a network and calculate first, last, mask, and host values. IP in CIDR Check whether a specific IP address belongs to a network. Subnet splitter Split a CIDR block into smaller child networks. Range to CIDR Convert start and end addresses into CIDR blocks.

FAQ

Is 8.8.8.8/24 the same as 8.8.8.0/24?

For network calculations, yes. 8.8.8.8/24 is host-prefix input that normalizes to the canonical network 8.8.8.0/24.

Does IPv6 use netmasks and broadcast addresses?

IPv6 uses prefix lengths, but IPv4-style netmasks and broadcast addresses are not used in the same way.