A /25 subnet mask means 25 bits are reserved for the network portion, leaving 7 bits for host addresses. In dotted decimal, that is: 11111111.11111111.11111111.10000000 Decimal equivalent: 255.255.255.128 A). 255.255.254.0 corresponds to /23. B). 255.255.255.1 is invalid as a subnet mask. D). 255.255.255.192 corresponds to /26. Thus, the correct subnet mask for a /25 network is 255.255.255.128. References (CompTIA Network+ N10-009): Domain: Networking Concepts - Subnetting, CIDR notation, dotted decimal.