Networking - Subnetting - Discussion

Discussion :: Subnetting - Subnetting (Q.No.18)

18. 

Which configuration command must be in effect to allow the use of 8 subnets if the Class C subnet mask is 255.255.255.224?

[A]. Router(config)#ip classless
[B]. Router(config)#no ip classful
[C]. Router(config)#ip unnumbered
[D]. Router(config)#ip subnet-zero

Answer: Option D

Explanation:

A Class C subnet mask of 255.255.255.224 is 3 bits on and 5 bits off (11100000) and provides 8 subnets, each with 30 hosts. However, if the command ip subnet-zero is not used, then only 6 subnets would be available for use.

Rohen said: (Dec 1, 2012)  
What is the following command I don't know I have seen this 1st time please tell me?

Router(config)#ip classless
Router(config)#no ip classful
Router(config)#ip unnumbered
Router(config)#ip subnet-zero

Idris said: (Jan 14, 2014)  
the first one is( Regardless of of class that is either class A,B or C). While the second one means you are not assigning IP to your router,

Third - have not come across that.

fourth - Assigning of ip without subnet masks Zero like 255.255.255.228 not 255.255.255.0.

Navin said: (Oct 11, 2014)  
255.255.255.224 already provides 8 subnets and 30 hosts each. Then, why there is need to use any other command.

The question and the answer also not clear ?
Where does 6 subnets come into this ?

Please justify !

Dave said: (Feb 2, 2015)  
A mask of 224 in the last octet means the block size of each subnet is 32.

So the 1st subnet is 0 through 32, 64, 96, 128, 160, 192, 224 which gives 8 subnets.

But with "no ip subnet-zero" you can't use the "0" subnet so you get 7 subnets not 6.

Maria said: (Aug 25, 2016)  
How come 6 subnets?

Because if we are not including 0 means, totally 7 subnets only will come right? Kindly clarify it.

Hardik Chugh said: (Oct 23, 2017)  
Because 1st is network address and last is a broadcast address which is not usable, hence 6 subnets.

Zakura said: (Oct 9, 2019)  
What if ;ip classful?

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