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New Member

Easy question on Subnet zero consideration, subnetting

Hi, can you help me with the basic thought below ?

I have been assigned network block.

I need to establish 8 subnets.

1) Assuming that the first subnet is the "0" subnet and

the last one is the broadcast one, I would do

2^n - 2 =

2^3 - 2 = 6 subnets

Therefore I would get 6 subnets if I use 3 bit from the Host portion. I need more.

Therefore I need 4 bit:

2^4 - 2 = 14 subnets - OK

Since I am using 4 bit, my subnet mask is:

/20 =

Increment = 256-240 = 16

Now here is my question.

Using this rationale that I discounted the "0" subnet and the broadcast

subnet by using the formula 2^2 - 2 ,which would be my very first subnet ?


or is it

Subnet ?

Also, what is the real implication of using the "zero" subnet ?

Let's say if I need to configure this in my router, it seems alright to me if I do:

#router rip


Can you explain why we don't normally consider the "0" subnet ? It seems functional to me.

New Member

Re: Easy question on Subnet zero consideration, subnetting

Your first subnet would be

You can use the subnet calculator to verify your calculation..

Hall of Fame Super Silver

Re: Easy question on Subnet zero consideration, subnetting


The restriction on subnet zero is very old - and is almost entirely not enforced in modern networking. The ip subnet-zero command is enabled by default in IOS and has been for a long time. As long as it is enabled then you should be able to use subnet zero with no particular impact.

In your situation if you need 8 subnets - and if you are confident that there will not be growth that might mean that you could need to support 9 or 10 subnets in that address space - then I would suggest that you use 3 bits for subnetting (the 248 mask) and get exactly the 8 subnets that you need.



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