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A Netmask is a 32-bit "mask" used to divide an IP address into subnets and specify the network's available hosts. In a netmask, two bits are always automatically assigned. For example, in 255.255.225.0, "0" is the assigned network address. In 255.255.255.255, "255" is the assigned broadcast address. The 0 and 255 are always assigned and cannot be used.
Netmask defines how "large" a network is or if you're configuring a rule that requires an IP address and a Netmask, the Netmask will signify to what range of the Network the rule will apply to:
| IP | NETMASK | DESCRIPTION |
|---|---|---|
| 192.168.55.161 | 255.255.255.255 | Only applies to 192.168.55.161 |
| 192.168.55.0 | 255.255.255.0 | Applies to IPs in the 192.168.55.0 - 192.168.55.255 range |
| 192.168.55.240 | 255.255.255.240 | 192.168.55.240 - 192.168.55.255 |
| 192.168.55.161 | 255.255.255.0 | 192.168.55.0 - 192.168.55.255 |
| 192.168.0.0 | 255.255.0.0 | 192.168.0.0 - 192.168.255.255 |
Sometimes you will see that a Netmask is defined by one number, e.g., 24. This number is the length of the Netmask in bits:
| NETMASK | 255. | 255. | 255. | 255 |
|---|---|---|---|---|
| Netmask length | 8 | 16 | 24 | 32 |
So, for example, a Netmask that is 24-bits long is 255.255.255.0:
| NETMASK | 255. | 255. | 255. | 0 |
|---|---|---|---|---|
| Netmask length | 8 | 16 | 24 | - |
A Netmask that is 16-bits long is 255.255.0.0:
| NETMASK | 255. | 255. | 0. | 0 |
|---|---|---|---|---|
| Netmask length | 8 | 16 | - | - |
And so on.