The concept of the role of the subnet mask and

①, subnet mask (Subnet Mask) called network mask address mask to be used in conjunction with the corresponding IP address.
②, only through the subnet mask, in order to show the subnet where a host of relationships with other subnets, make the network work.
③, subnet mask, and IP addresses do "and" operation, and separating the host address of a network address in the IP address, for determining whether the IP address on the local network, remote network or the Internet.
④, the subnet mask is also used to further divide the network into several subnets, hosts and to avoid excessive or too little congestion and IP waste.