# Netmask

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](/glossary/i/ipv4_broadcast) address.