subnet network mask - The network
mask is usually 255.0.0.0 for a class A network, 255.255.0.0 for a
class B network and 255.255.255.0 for a class C network. The network
mask controls how many addresses the subnet contains. For a class C
network, all IP addresses are available for use except for x.x.x.0 and
x.x.x.255.