An IP subnet mask of 27 bits means 32 - 27 = 5 bits are left for host addresses. The number of hosts per subnet is 2^5 - 2 = 30. The subtraction of 2 accounts for the network address and broadcast address, which cannot be assigned to hosts. This calculation is fundamental in IP subnetting and is widely used in network design to allocate IP address ranges efficiently. HCIP-Transmission materials include these principles as part of the networking fundamentals necessary for transmission network engineers.