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Subnet mask is a binary number associated with a TCP/IP address. It allows a computer to distinguish which parts in an IP address correspond to the network ID and host ID portions of the address. In binary representation of subnet mask, the bits set to 1 correspond to the network ID, and the bits set to 0 correspond to host ID. All IP interfaces on the same network segment must use the same network ID and subnet mask. The subnet mask is a required configuration item.