Archive for the Ip Subnet-zero Category

IP Subnet zero

Posted in Ip Subnet-zero on February 12, 2009 by itdaddy

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.

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