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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.

Last reviewed: June 12, 2026

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Quick reference

Common prefixes

IPv4 /32
One IPv4 address.
IPv4 /24
256 IPv4 addresses, commonly written with netmask 255.255.255.0.
IPv4 /16
65,536 IPv4 addresses.
IPv6 /128
One IPv6 address.
IPv6 /64
Common size for a single IPv6 LAN or subnet.
IPv6 /48
Common site-level IPv6 allocation size in many designs.

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Guide

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.

Guide

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.

Reference

Key terms

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

Examples

Examples

Next steps

Related guides and tools

CIDR prefix cheat sheet Scan common IPv4 and IPv6 prefix sizes. 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.

Questions

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.

Sources

References

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