The Subnet Mask is a number that identifies a set of IP addresses that can be used on a network. Subnet masks are often used to denote subnets that are local network LANs connected to the Internet. Systems in the same subnet can communicate directly with each other, while systems in different subnets must communicate through a router. Therefore, subnets can be used to divide multiple networks and limit traffic between them.

A "Subnet Mask" hides or "masks" the network portion of the system's IP address, leaving only the host portion as the machine identifier. Generally, a subnet mask is set to 255.255.255.0 (not mandatory). Each part of the subnet mask can contain a number between 0 and 255, just like an IP address. So, in the example above, the first three sections are occupied, meaning that the IP addresses of the computers in the subnet mask must be the same in the first three sections.

Although it is not mandatory, the reason for assigning 255 is under 1 because it cannot be 256, because this is the maximum number that can be assigned and it is desirable not to overlap in any case.