What is a Netmask?

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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, "0" is the assigned network address. In, "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 Only applies to Applies to IPs in the - range - - -

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

NETMASK 255. 255. 255. 0
Netmask length 8 16 24 -

A Netmask that is 16-bits long is

NETMASK 255. 255. 0. 0
Netmask length 8 16 - -

And so on.