Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
A subnet mask specifies how many bits belong to the network portion of a 32bit IP address. The remaining bits in the IP address belong to the host portion of the IP address. To determine how many host addresses are defined by a subnet mask, use the formula 2n-2, where n is the number of bits in the host portion of the address.
A /19 subnet mask uses 13 bits for host addresses. Therefore, 213 -2 equals 8,190 valid host addresses.
A /20 subnet mask uses 12 bits for host addresses. Therefore, 212 -2 equals 4,094 valid host addresses.
A /22 subnet mask uses 10 bits for host addresses. Therefore, 210-2 equals 1,022 valid host addresses.
A /23 subnet mask uses nine bits for host addresses. Therefore, 29-2 equals 510 valid host addresses.
A /25 subnet mask uses seven bits for host addresses. Therefore, 27-2 equals 126 valid host addresses.
Although it is important to learn the formula for calculating valid host addresses, the following list demonstrates the relationship between subnet masks and valid host addresses:

Reference:
CCDA 200-310 Official Cert Guide, Chapter 8, IPv4 Address Subnets, pp. 302-310 Cisco: IP Addressing and Subnetting for New Users