Note: If this is a network, and you plan on creating subnetworks within it, then you must specify a netmask here. If this is a subnetwork, or if this is a network in which you will not be creating any subnetworks, you can leave this field empty.

A netmask is a number in IP address format that constrains the numerical range of IP addresses available to computers on a network. For example, a netmask of 255.255.0.0 means that IP addresses on the network must match the first two segment groups exactly, but will match any number in the last two segments.

For example, in a network with an IP address of 130.155 and a netmask of 255.255.0.0, the IP addresses for all computers on that network must begin with 130.155, but then can use any number from 0 through 255 for the last two segments of the IP address.

Any computer's IP address that does not match the netmask must use a gateway computer to connect to the network.