Jack, a network technician, needs to subnet a class C network into as many subnets as possible, with at least 22 usable hosts per subnet. What subnet mask should he use?
Answer Description
255.255.255.224 is the correct subnet, because it gives 30 usable hosts per subnet, and will give you the most possible subnets.
Wikipedia
A subnetwork or subnet is a logical subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting.
Computers that belong to a subnet are addressed with a common, identical, most-significant bit-group in their IP address. This results in the logical division of an IP address into two fields, a network or routing prefix and the "rest" field or host identifier. The rest field is an identifier for …
Subnetwork - Wikipedia, the free encyclopedia