Published at LXer:

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.

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