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CIDR Reference Table
Every CIDR prefix from /1 to /32 — subnet mask, wildcard, total addresses, and usable hosts
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| Prefix | Subnet Mask | Wildcard | Addresses | Usable Hosts | Class |
|---|---|---|---|---|---|
| /0 | 0.0.0.0 | 255.255.255.255 | 4,294,967,296 | 4,294,967,294 | A |
| /1 | 128.0.0.0 | 127.255.255.255 | 2,147,483,648 | 2,147,483,646 | A |
| /2 | 192.0.0.0 | 63.255.255.255 | 1,073,741,824 | 1,073,741,822 | A |
| /3 | 224.0.0.0 | 31.255.255.255 | 536,870,912 | 536,870,910 | A |
| /4 | 240.0.0.0 | 15.255.255.255 | 268,435,456 | 268,435,454 | A |
| /5 | 248.0.0.0 | 7.255.255.255 | 134,217,728 | 134,217,726 | A |
| /6 | 252.0.0.0 | 3.255.255.255 | 67,108,864 | 67,108,862 | A |
| /7 | 254.0.0.0 | 1.255.255.255 | 33,554,432 | 33,554,430 | A |
| /8 | 255.0.0.0 | 0.255.255.255 | 16,777,216 | 16,777,214 | A |
| /9 | 255.128.0.0 | 0.127.255.255 | 8,388,608 | 8,388,606 | B |
| /10 | 255.192.0.0 | 0.63.255.255 | 4,194,304 | 4,194,302 | B |
| /11 | 255.224.0.0 | 0.31.255.255 | 2,097,152 | 2,097,150 | B |
| /12 | 255.240.0.0 | 0.15.255.255 | 1,048,576 | 1,048,574 | B |
| /13 | 255.248.0.0 | 0.7.255.255 | 524,288 | 524,286 | B |
| /14 | 255.252.0.0 | 0.3.255.255 | 262,144 | 262,142 | B |
| /15 | 255.254.0.0 | 0.1.255.255 | 131,072 | 131,070 | B |
| /16 | 255.255.0.0 | 0.0.255.255 | 65,536 | 65,534 | B |
| /17 | 255.255.128.0 | 0.0.127.255 | 32,768 | 32,766 | C |
| /18 | 255.255.192.0 | 0.0.63.255 | 16,384 | 16,382 | C |
| /19 | 255.255.224.0 | 0.0.31.255 | 8,192 | 8,190 | C |
| /20 | 255.255.240.0 | 0.0.15.255 | 4,096 | 4,094 | C |
| /21 | 255.255.248.0 | 0.0.7.255 | 2,048 | 2,046 | C |
| /22 | 255.255.252.0 | 0.0.3.255 | 1,024 | 1,022 | C |
| /23 | 255.255.254.0 | 0.0.1.255 | 512 | 510 | C |
| /24 | 255.255.255.0 | 0.0.0.255 | 256 | 254 | C |
| /25 | 255.255.255.128 | 0.0.0.127 | 128 | 126 | Subnetting |
| /26 | 255.255.255.192 | 0.0.0.63 | 64 | 62 | Subnetting |
| /27 | 255.255.255.224 | 0.0.0.31 | 32 | 30 | Subnetting |
| /28 | 255.255.255.240 | 0.0.0.15 | 16 | 14 | Subnetting |
| /29 | 255.255.255.248 | 0.0.0.7 | 8 | 6 | Subnetting |
| /30 | 255.255.255.252 | 0.0.0.3 | 4 | 2 | Subnetting |
| /31 | 255.255.255.254 | 0.0.0.1 | 2 | 2 | Subnetting |
| /32 | 255.255.255.255 | 0.0.0.0 | 1 | 1 | Subnetting |
Showing 33 entries · /31 has 2 usable (point-to-point links per RFC 3021) · /32 is a host route
The most important prefixes to memorise
/8
255.0.0.0
16,777,214 hosts · Class A default
/16
255.255.0.0
65,534 hosts · Class B default
/24
255.255.255.0
254 hosts · Class C default — most common
/25
255.255.255.128
126 hosts · First common subnetting split
/26
255.255.255.192
62 hosts · 4 subnets from a /24
/27
255.255.255.224
30 hosts · 8 subnets from a /24
/28
255.255.255.240
14 hosts · Common small LAN
/30
255.255.255.252
2 hosts · Point-to-point WAN links