IP Subnet Zero

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 2^N 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 2^N rather than 2^N 2.

2N	Number of total subnets created
2N 2	Number of valid subnets created	No longer needed because you have the ip subnet-zero command enabled
2H	Number of total hosts per subnet
2H 2	Number of valid hosts per subnet