Subnet mask is also called network mask, address mask and subnet mask. It is used to indicate which bits of an IP address identify the subnet where the host is located and which bits identify the bit mask of the host. The subnet mask cannot exist alone. It must be used in combination with the IP address. The subnet mask has only one function, which is to divide an IP address into two parts: network address and host address. The subnet mask is the basic knowledge that everyone who uses the Internet must master. Only by mastering it can you truly understand the TCP/IP protocol settings. Let's explain what subnet mask is in simple terms.