About
subnet mask is the second part of an Ip address. It's also called the mask of a subnet because all hosts on a subnet have the same network mask.
The maximum size of the network is given by the number of addresses that are possible with the remaining, least-significant bits in the mask below the prefix. See table
Notation
Mask can be defined via:
Dot-decimal
The dot-decimal notation separated the byte with a dot converted in decimal.
Example in ipv4
255.255.255.000
Prefix
The (routing) prefix is the number of leading 1 bits.
Example with an ipv4 address
11111111 00000000 00000000 00000000 = 8 leading 1 bits = prefix of /8
Larger values indicate smaller networks. See the table below.
The prefix length is used in the CIDR notation.
Binary
Example:
1111111 0000000 0000000 000000
Table
| Prefix size | Network mask | Usable hosts per subnet |
|---|---|---|
| /1 | 128.0.0.0 | 2,147,483,646 |
| /2 | 192.0.0.0 | 1,073,741,822 |
| /3 | 224.0.0.0 | 536,870,910 |
| /4 | 240.0.0.0 | 268,435,454 |
| /5 | 248.0.0.0 | 134,217,726 |
| /6 | 252.0.0.0 | 67,108,862 |
| /7 | 254.0.0.0 | 33,554,430 |
| Class A | ||
| /8 | 255.0.0.0 | 16,777,214 |
| /9 | 255.128.0.0 | 8,388,606 |
| /10 | 255.192.0.0 | 4,194,302 |
| /11 | 255.224.0.0 | 2,097,150 |
| /12 | 255.240.0.0 | 1,048,574 |
| /13 | 255.248.0.0 | 524,286 |
| /14 | 255.252.0.0 | 262,142 |
| /15 | 255.254.0.0 | 131,070 |
| Class B | ||
| /16 | 255.255.0.0 | 65,534 |
| /17 | 255.255.128.0 | 32,766 |
| /18 | 255.255.192.0 | 16,382 |
| /19 | 255.255.224.0 | 8,190 |
| /20 | 255.255.240.0 | 4,094 |
| /21 | 255.255.248.0 | 2,046 |
| /22 | 255.255.252.0 | 1,022 |
| /23 | 255.255.254.0 | 510 |
| Class C | ||
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /27 | 255.255.255.224 | 30 |
| /28 | 255.255.255.240 | 14 |
| /29 | 255.255.255.248 | 6 |
| /30 | 255.255.255.252 | 2 |
| /31 | 255.255.255.254 | 0 |
| /32 | 255.255.255.255 | 0 |