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Calculating the Subnets.

jskfan
jskfan asked
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Last Modified: 2019-01-26
Calculating the Subnets.

I am using the online Subnet Calculator.  Some Data displayed on the calculator and indicated by the Red arrows on the screenshot below  need some explanation:

example:
how did they come with subnet bits: 12
Maximum subnets 4096
host per subnets :14
subnet ID :172.16.1.0
Broadcast address : 172.16.1.15

Thank you


subnet.JPG
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Sr. Network Engineer
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Don JohnstonInstructor
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Top Expert 2015

Commented:
how did they come with subnet bits: 12

You start with an address that has a 16-bit mask.   You extend the mask to accommodate the number of subnets/hosts and you end up with a 28-bit mask.  

28-16=12.  

Actual mask minus the original (or starting) mask will give the size of the subnet field.  Which in this case is 12.
SouljaSr.Net.Eng
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Top Expert 2011

Commented:
You chose class B network mask, so your results of a /28 is 12 bits in from the /16. Now if you go back and choose class C network, you will only be 4 bits from the /24.

Author

Commented:
atlas,

can you clarify this :
Your first usable subnet will be 172.16.0.0/18.
atlas_shudderedSr. Network Engineer
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Commented:
jsk -

Sorry - should say

Your first usable subnet will be 172.16.0.0/28.
First address in the subnet is 172.16.0.0
Last address in the subnet is 172.16.0.15
First address in a subnet is always the network address and unusable (subnet ID)
Last address in a subnet is always the broadcast address and unusable (Broadcast Address)

Does that clarify?

Author

Commented:
Thank you for clear explanation