A netmask is a mask used to divide an IP address into subnets and specify the number of available host addresses in the subnet.
The netmask consists of a 32-bit sequence on the format 255.255.225.255.
There are three commonly used network classes:
| Class | Netmask length | # of networks | Number of hosts | Netmask |
|---|---|---|---|---|
| Class A | 8 | 126 | 16777214 | 255.0.0.0 |
| Class B | 16 | 16382 | 65534 | 255.255.0.0 |
| Class C | 24 | 2097150 | 254 | 255.255.255.0 |
The last 0 and 255 in a sequence are always assigned and cannot be used as host addresses.
For example, in 255.255.225.0, 0 is the assigned network address.
In 255.255.255.255, 255 is the assigned IPv4 broadcast address.