View source for What is a Netmask?

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 portion or range of the Network the rule will apply to:

{| class="wikitable"
|+
! style="width: 200px; background: black; color: white;"  | IP
! style="width: 200px; background: black; color: white;"  | Netmask
! style="width: 200px; background: black; color: white;" | Description
|-
! style="text-align: left; vertical-align: top;" | 192.168.55.161
| style="text-align: left; vertical-align: top;" | 255.255.255.255
| style="text-align: left; vertical-align: top;" | Only applies to 192.168.55.161'''
|-
! style="text-align: left; vertical-align: top;" | 192.168.55.0
| style="text-align: left; vertical-align: top;" | 255.255.255.0
| style="text-align: left; vertical-align: top;" | Applies to IPs in the 192.168.55.0 - 192.168.55.255 range
|-
! style="text-align: left; vertical-align: top;" | 192.168.55.240
| style="text-align: left; vertical-align: top;" | 255.255.255.240
| style="text-align: left; vertical-align: top;" | 192.168.55.240 - 192.168.55.255
|-
! style="text-align: left; vertical-align: top;" | 192.168.55.161
| style="text-align: left; vertical-align: top;" | 255.255.255.0
| style="text-align: left; vertical-align: top;" | 192.168.55.0 - 192.168.55.255
|-
! style="text-align: left; vertical-align: top;" | 192.168.0.0
| style="text-align: left; vertical-align: top;" | 255.255.0.0
| style="text-align: left; vertical-align: top;" | 192.168.0.0 - 192.168.255.255
|-
|}

Sometimes you will see a Netmask that is defined by one number, e.g., '''24'''. This number is a the length of the Netmask in bits:

{| class="wikitable"
|+
! style="width: 200px; background: black; color: white;" | Netmask
! style="width: 200px; background: black; color: white;" | 255.
! style="width: 200px; background: black; color: white;" | 255.
! style="width: 200px; background: black; color: white;" | 255.
! style="width: 200px; background: black; color: white;" | 255
|-
! style="text-align: left; vertical-align: top;"   | Netmask length
| style="text-align: center; vertical-align: top;" | 8
| style="text-align: center; vertical-align: top;" | 16
| style="text-align: center; vertical-align: top;" | 24
| style="text-align: center; vertical-align: top;" | 32
|-
|}


So, for example, a Netmask that is 24-bits long is '''255.255.255.0''':

{| class="wikitable"
|+
! style="width: 200px; background: black; color: white;" | Netmask
! style="width: 200px; background: black; color: white;" | 255.
! style="width: 200px; background: black; color: white;" | 255.
! style="width: 200px; background: black; color: white;" | 255.
! style="width: 200px; background: black; color: white;" | 0
|-
! style="text-align: left; vertical-align: top;"   | Netmask length
| style="text-align: center; vertical-align: top;" | 8
| style="text-align: center; vertical-align: top;" | 16
| style="text-align: center; vertical-align: top;" | 24
| style="text-align: center; vertical-align: top;" | -
|-
|}


A Netmask that is 16-bits long is '''255.255.0.0''':

{| class="wikitable"
|+
! style="width: 200px; background: black; color: white;" | Netmask
! style="width: 200px; background: black; color: white;" | 255.
! style="width: 200px; background: black; color: white;" | 255.
! style="width: 200px; background: black; color: white;" | 0
! style="width: 200px; background: black; color: white;" | 0
|-
! style="text-align: left; vertical-align: top;"   | Netmask length
| style="text-align: center; vertical-align: top;" | 8
| style="text-align: center; vertical-align: top;" | 16
| style="text-align: center; vertical-align: top;" | -
| style="text-align: center; vertical-align: top;" | -
|-
|}

And so on.