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:
|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:
So, for example, a Netmask that is 24-bits long is 255.255.255.0:
A Netmask that is 16-bits long is 255.255.0.0:
And so on.