IP Subnet Zero

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When you work with classical subnetting, you always have to eliminate the subnets that contain either all zeros or all ones in the subnet portion. Hence, you always used the formula 2N 2 to define the number of valid subnets created. However, Cisco devices can use those subnets, as long as the command ip subnet-zero is in the configuration. This command is on by default in Cisco IOS Software Release 12.0 and later; if it was turned off for some reason, however, you can re-enable it by using the following command:
Router(config)#ip subnet-zero
 Now you can use the formula 2N rather than 2N - 2.
2NNumber of total subnets created 
2N 2Number of valid subnets createdNo longer needed because you have the ip subnet-zero command enabled
2HNumber of total hosts per subnet 
2H 2Number of valid hosts per subnet 

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