Use Gip For IP Address Calculations In Ubuntu/Linux Mint
Subnetting in a nutshell is a logical division of an IP network. It is done to provide a better network management, for security purposes or to simplify the network topology. This is necessary for system administrators, network engineers, database administrators and so on.
Understanding how to subnet is a detailed topic. Here are a few excellent resources to start with :
IP Addressing and Subnetting...