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:

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 a Netmask that is defined by one number, e.g., 24. This number is a 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	-