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 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.


Number of total subnets created


2N 2

Number of valid subnets created

No longer needed because you have the ip subnet-zero command enabled


Number of total hosts per subnet


2H 2

Number of valid hosts per subnet


CCNA Self-Study(c) CCNA Portable Command Guide
CCNA Portable Command Guide
ISBN: 1587201585
EAN: 2147483647
Year: 2006
Pages: 261
Authors: Scott Empson

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