Reliable Subnet Calculator - Version 1.1 Reliable Subnet Calculator is a security solution that allows one to quickly calculate subnet mask for a given IP address range. Necessity to accurately calculate subnet mask can be a very tedious task for individuals lacking thorough understanding of IP addressing, subnetting and other networking concepts. Reliable Subnet Calculator is extremely useful in situations when one needs to quickly block entire IP address range from accessing specific resources e.g. IIS website or virtual directory.