guest@ipterm:~$ cidr --table
Subnet mask cheat sheet: CIDR to netmask
CIDR notation writes a network as an address plus a prefix length: 192.168.1.0/24 means the first 24 bits identify the network and the remaining 8 identify hosts. This page converts every common prefix to its subnet mask.
Reading CIDR notation
- Prefix length is the number of 1 bits in the subnet mask: /24 is 255.255.255.0 because the first 24 bits are 1.
- Addresses in a block are 2(32 − prefix): a /24 has 256.
- Usable hosts subtract the network address (the first) and the broadcast address (the last). Two exceptions: a /31 has two usable addresses for point-to-point links (RFC 3021), and a /32 is a single host.
- Wildcard mask is the netmask inverted, used in router access lists and some firewall rules.
IPv4 prefixes from /8 to /32
| Prefix | Subnet mask | Wildcard | Addresses | Usable hosts |
|---|---|---|---|---|
| /8 | 255.0.0.0 | 0.255.255.255 | 16,777,216 | 16,777,214 |
| /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 |
| /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 |
| /17 | 255.255.128.0 | 0.0.127.255 | 32,768 | 32,766 |
| /18 | 255.255.192.0 | 0.0.63.255 | 16,384 | 16,382 |
| /19 | 255.255.224.0 | 0.0.31.255 | 8,192 | 8,190 |
| /20 | 255.255.240.0 | 0.0.15.255 | 4,096 | 4,094 |
| /21 | 255.255.248.0 | 0.0.7.255 | 2,048 | 2,046 |
| /22 | 255.255.252.0 | 0.0.3.255 | 1,024 | 1,022 |
| /23 | 255.255.254.0 | 0.0.1.255 | 512 | 510 |
| /24 | 255.255.255.0 | 0.0.0.255 | 256 | 254 |
| /25 | 255.255.255.128 | 0.0.0.127 | 128 | 126 |
| /26 | 255.255.255.192 | 0.0.0.63 | 64 | 62 |
| /27 | 255.255.255.224 | 0.0.0.31 | 32 | 30 |
| /28 | 255.255.255.240 | 0.0.0.15 | 16 | 14 |
| /29 | 255.255.255.248 | 0.0.0.7 | 8 | 6 |
| /30 | 255.255.255.252 | 0.0.0.3 | 4 | 2 |
| /31 | 255.255.255.254 | 0.0.0.1 | 2 | 2 |
| /32 | 255.255.255.255 | 0.0.0.0 | 1 | 1 |
Common IPv6 prefix sizes
| Prefix | Typical use |
|---|---|
| /128 | A single address |
| /64 | One local network. Address autoconfiguration (SLAAC) expects exactly a /64. |
| /56 | A common home allocation: 256 networks of /64 |
| /48 | A common site or business allocation: 65,536 networks of /64 |
| /32 | A typical minimum allocation to an internet provider from a regional registry |
Try itRun cidr 10.0.0.0/22, or any range, in the terminal for the full breakdown, including which bits are network and which are host.