ip subnet-zero: Cisco IOS command using with examples

ip subnet-zero: Cisco command
Andrey Volkov

Andrey Volkov

System, network administration + DBA. And a little programmer!)) See Author profile.

Historically, it was always recommended that a subnet of all 0s or a subnet of all 1s not be used. Therefore, the formula of 2N – 2 was used to calculate 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 reenable 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

 

Note

All of this is explained in great detail in RFC 950, Internet Standard for Subnetting Procedure (August 1985).

 

Note

RFC 1878, Variable Length Subnet Table for IPv4 (December 1995), states, “This practice [of excluding all-zeros and all-ones subnets] is obsolete. Modern software will be able to utilize all definable networks.”

 

Вас заинтересует / Intresting for you:

Cisco: Shortcuts for Entering ...
Cisco: Shortcuts for Entering ... 311 views Андрей Волков Wed, 17 Feb 2021, 15:59:18
Cisco: Resetting Switch Config...
Cisco: Resetting Switch Config... 334 views Андрей Волков Mon, 15 Feb 2021, 19:13:09
Cisco ASA 5500-X: Reimaging Es...
Cisco ASA 5500-X: Reimaging Es... 2087 views Андрей Волков Sun, 10 Nov 2019, 14:34:36
Cisco: Console Error Messages
Cisco: Console Error Messages 306 views Андрей Волков Thu, 18 Feb 2021, 18:04:00