When a network address is subnetted, the first subnet obtained after subnetting the network address is called subnet zero, whereas the last subnet obtained is called the all-ones subnet.

Typically in the past you were advised not to use these subnets for nodes.

Today we are free to use them, but we need to remember to make sure ip subnet zero is turned on in the router configuration. ip subnet zero is a special command on Cisco routers which enables you to take advantage of these extra subnets, and since version 12 of Cisco IOS this command has been switched on by default.

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