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Guide

CIDR Prefix Cheat Sheet

CIDR prefix lengths get easier when you can map common slash numbers to address counts and practical network sizes.

Last reviewed: June 12, 2026

Calculate a CIDR prefix

Quick reference

Common IPv4 prefixes

/32
1 address. Often used for a single host allowlist rule.
/30
4 addresses, 2 traditionally usable host addresses.
/29
8 addresses, often seen in small provider allocations.
/24
256 addresses, netmask 255.255.255.0.
/16
65,536 addresses, common as a large private network boundary.
/8
16,777,216 addresses.

Quick reference

Common IPv6 prefixes

/128
1 IPv6 address.
/64
Common size for one IPv6 LAN or subnet.
/56
Often used as a residential or small-site delegation size.
/48
Often used as a site allocation size.
/32
Common size for a large provider or documentation-level block.

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Guide

How to use the table

Use the cheat sheet to recognize common prefix sizes quickly, then use the CIDR calculator when you need exact first address, last address, broadcast, netmask, or total count.

Do not infer ownership from a prefix alone. A CIDR block is a range format; ASN and routing data explain who appears to operate a public network.

Guide

Host-prefix reminder

A value such as 8.8.8.8/24 contains host bits. Network tools usually normalize it to the canonical network 8.8.8.0/24 before calculating the range.

Reference

Key terms

Prefix
Slash length such as /24 or /64
Netmask
IPv4 dotted-decimal mask for a prefix
Broadcast
IPv4 address used for broadcast in traditional subnetting

Examples

Examples

Small IPv4 subnet

203.0.113.0/29 Eight IPv4 addresses. Calculate /29

IPv6 LAN

2001:db8:1::/64 One common IPv6 subnet size. Calculate /64

Next steps

Related guides and tools

CIDR notation explained Learn what prefix lengths and host bits mean. CIDR calculator Calculate exact network details for any valid IPv4 or IPv6 prefix. Subnet splitter Split a parent network into child prefixes. Range to CIDR Convert start/end ranges into covering CIDR blocks.

Questions

FAQ

Why does a larger slash number mean fewer addresses?

The prefix fixes more leading bits as the network part, leaving fewer remaining bits for addresses inside the block.

Are IPv6 /64 networks always required?

Not for every use, but /64 is the standard size for many IPv6 LAN designs and features such as SLAAC.

Sources

References

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