The most efficient subnet address for a network that will have no more than 30 available IP addresses is 255.255.255.224. This subnet mask corresponds to a /27 prefix length, which means that 27 bits are used for the network portion and 5 bits are used for the host portion of an IP address. With 5 bits for hosts, there are 2^5 - 2 = 30 possible host addresses per subnet, which meets the requirement. The other options are either too large or too small for the network size. Reference: https://www.ibm.com/cloud/learn/subnet-mask