"Setting up a network can be fairly quick and easy if you're
setting it up with straight Class A, Class B, or Class C network
addresses. But, there may be occasions when you would only have a
few hosts assigned to a given network segment. In that case, you
wouldn't want to use a straight 'classful' network addressing
scheme, because you would waste too many IP addresses for that
network. (You may need to use these IP addresses later if you
decide to add other segments to your network.) This is where
subnetting comes in handy.
"When you subnet, you're dividing your Class A, Class B, or
Class C network into two or more smaller networks..."