12-07-2006 08:17 AM - edited 03-03-2019 02:57 PM
Hi, can you help me with the basic thought below ?
I have been assigned 132.45.0.0/16 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 = 255.255.240.0
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 ?
Subnet 132.45.16.0
or is it
Subnet 132.45.0.0 ?
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
network 132.45.0.0
Can you explain why we don't normally consider the "0" subnet ? It seems functional to me.
12-07-2006 09:29 AM
Your first subnet would be 132.45.16.0.
You can use the subnet calculator to verify your calculation..
http://www.wildpackets.com/products/free_utilities/ipsubnetcalc/overview
12-07-2006 09:37 AM
Marlon
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.
HTH
Rick
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