To understand what a subnet mask is, it is necessary to know first what an IP address is — this is the identifier of each device that connects to the network so that they can interact with each other. The IP address looks like a string of numbers separated by dots, for example, 172.16.81.100.
The subnet mask is also a number, but it defines the range of IP addresses that the network can use. With its help, networks can be divided into small subnets that connect to the Internet. Simply put, the task of a subnet mask is to hide the network element of the address so that only the host element remains visible. The mask looks like this: 255.255.255.0. But the subnet mask is not part of the IP packet, so upon decryption of the IP address online this number will not be displayed.