CIDR Cheat Sheet: Every Prefix From /0 to /32

You will look up the same handful of numbers over and over when you work with networks: how many hosts in a /27, what mask goes with /22, is a /29 big enough. This page is the answer sheet. Bookmark it, or use our subnet calculator when you need the exact range for a specific address.

How to read a prefix in ten seconds

An IPv4 address has 32 bits. The number after the slash says how many of those bits belong to the network. The rest belong to the hosts on it.

• Bigger number, smaller network. A /30 is tiny, a /8 is enormous.
• Each step doubles or halves the size. A /23 is two /24s. A /25 is half a /24.
• Usable hosts are total minus two for every prefix up to /30, because the first address names the network and the last is the broadcast address.

That is genuinely all there is to it. For the slower, plain English walk through, see subnet mask explained.

The full table: /0 to /32

The rows you will actually use day to day are /16 to /30. Two shortcuts make them easy to remember: a /24 is 256 addresses, and every step away from /24 doubles or halves it. So a /26 is 64, a /27 is 32, a /28 is 16, and a /22 is 1,024.

Quick picks: which size for which job

In the cloud, subtract the provider's reserved addresses too. A /28 in AWS has 16 addresses, minus 5 reserved, which leaves 11 for your instances.

Cloud subnet rules

AWS's default VPC in every region uses 172.31.0.0/16. If you plan to connect cloud networks to your office over a VPN, pick ranges that do not overlap with your office network or with each other. Overlapping private ranges are the single most common reason site to site VPNs refuse to route.

Ranges that are reserved for something

If you see one of the private ranges as your address on a device, that is your local address, not the one the internet sees. Our home page shows your public one.

What sizes do registries actually hand out?

We counted every allocated or assigned block in the files that the five regional internet registries (AFRINIC, APNIC, ARIN, LACNIC and RIPE NCC) publish, dated 22 and 23 October 2026.

There are 262,321 IPv4 entries in total. The /22 is so common because that was the size RIPE NCC and APNIC handed out from their final pools as IPv4 ran out. The /24 matters for another reason: it is the smallest block most networks will accept in internet routing, so anything smaller than a /24 cannot be announced on its own.

On the IPv6 side, out of 70,981 entries, 46.2% are a /32, 23.2% a /29 and 21.5% a /48. A /32 is the standard first allocation for an ISP, and a /48 is the usual size for a single organisation or site.

IPv6 prefixes in one table

The rule that trips people up: always give a LAN a full /64, even if it only has three devices. IPv6 address autoconfiguration (SLAAC) expects 64 bits for the host part and breaks on anything smaller. Want to see whether you even have IPv6 right now? Run our IPv6 test, and see how your country compares in our IPv6 adoption by country data.

The bottom line

A /24 is 256 addresses and 254 hosts, and every step away from it doubles or halves that. Keep the full table above for everything else. Add the cloud rule (AWS and Azure take 5 addresses per subnet, Google Cloud takes 4), keep private ranges from overlapping, and give every IPv6 LAN a /64. For a specific address, our subnet calculator gives you the network, broadcast and host range instantly.