How to read the table
The prefix length is the number of network bits. Each step down the table halves the block: a /23 is two /24s, a /22 is four. Usable hosts are the total minus the network and broadcast addresses, except /31 (both usable on point-to-point links) and /32 (one host). The AWS column subtracts the five addresses AWS reserves in every subnet; Azure reserves five as well and Google Cloud four.
| Prefix | Subnet mask | Wildcard | Addresses | Usable | AWS usable | Typical use |
|---|---|---|---|---|---|---|
| /8 | 255.0.0.0 | 0.255.255.255 | 16,777,216 | 16,777,214 | — | Whole private range 10/8 |
| /9 | 255.128.0.0 | 0.127.255.255 | 8,388,608 | 8,388,606 | — | |
| /10 | 255.192.0.0 | 0.63.255.255 | 4,194,304 | 4,194,302 | — | |
| /11 | 255.224.0.0 | 0.31.255.255 | 2,097,152 | 2,097,150 | — | |
| /12 | 255.240.0.0 | 0.15.255.255 | 1,048,576 | 1,048,574 | — | Private range 172.16/12, Docker |
| /13 | 255.248.0.0 | 0.7.255.255 | 524,288 | 524,286 | — | |
| /14 | 255.252.0.0 | 0.3.255.255 | 262,144 | 262,142 | — | |
| /15 | 255.254.0.0 | 0.1.255.255 | 131,072 | 131,070 | — | |
| /16 | 255.255.0.0 | 0.0.255.255 | 65,536 | 65,534 | 65,531 | Largest AWS VPC; 192.168/16 |
| /17 | 255.255.128.0 | 0.0.127.255 | 32,768 | 32,766 | 32,763 | |
| /18 | 255.255.192.0 | 0.0.63.255 | 16,384 | 16,382 | 16,379 | |
| /19 | 255.255.224.0 | 0.0.31.255 | 8,192 | 8,190 | 8,187 | Per-zone slice of a VPC |
| /20 | 255.255.240.0 | 0.0.15.255 | 4,096 | 4,094 | 4,091 | AWS default-VPC subnet |
| /21 | 255.255.248.0 | 0.0.7.255 | 2,048 | 2,046 | 2,043 | |
| /22 | 255.255.252.0 | 0.0.3.255 | 1,024 | 1,022 | 1,019 | Office network, node subnet |
| /23 | 255.255.254.0 | 0.0.1.255 | 512 | 510 | 507 | |
| /24 | 255.255.255.0 | 0.0.0.255 | 256 | 254 | 251 | Classic LAN, home router |
| /25 | 255.255.255.128 | 0.0.0.127 | 128 | 126 | 123 | |
| /26 | 255.255.255.192 | 0.0.0.63 | 64 | 62 | 59 | Quarter of a /24 |
| /27 | 255.255.255.224 | 0.0.0.31 | 32 | 30 | 27 | Small server or NAT subnet |
| /28 | 255.255.255.240 | 0.0.0.15 | 16 | 14 | 11 | Smallest AWS subnet |
| /29 | 255.255.255.248 | 0.0.0.7 | 8 | 6 | — | ISP business block |
| /30 | 255.255.255.252 | 0.0.0.3 | 4 | 2 | — | Router link (legacy) |
| /31 | 255.255.255.254 | 0.0.0.1 | 2 | 2 | — | Router link (RFC 3021) |
| /32 | 255.255.255.255 | 0.0.0.0 | 1 | 1 | — | Single host |
Also see 0.0.0.0/0, the default route that matches every address.
FAQ
What does the number after the slash mean?
The prefix length: how many of the 32 bits of an IPv4 address identify the network. The remaining bits identify hosts, so a /24 leaves 8 host bits and 256 addresses.
How do I calculate usable hosts from a prefix?
Usable hosts = 2^(32 − prefix) − 2. The two removed addresses are the network address and the broadcast address. /31 and /32 are the exceptions, with 2 and 1 usable addresses.
Is /24 bigger than /16?
No. A smaller prefix number means more host bits and a bigger block. A /16 has 65,536 addresses; a /24 has 256.
Why does AWS show fewer usable IPs than the formula?
AWS reserves five addresses in every subnet (the first four and the last), so a /24 subnet offers 251 addresses. Azure also reserves five; Google Cloud reserves four.
Need a specific block? Open the CIDR / Subnet Calculator or read CIDR and Subnet Calculation Explained.