Subnet Mask

The subnet mask gives the router the information required to determine in which network and subnet a
particular host resides.
The subnet mask is created by using binary ones in the network and subnet bits, and zeros in the host bits.Class A = 255.0.0.0
Class B = 255.255.0.0
Class C = 255.255.255.0
If three bits were borrowed, the mask for a Class C address would be
255.255.255.224 or
255.255.255.224/27

The fourth octet, 224 = 11100000