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simple subnetting question

Posted on 2003-12-04
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Last Modified: 2010-03-19
In the following site http://www.learntosubnet.com/Subnetting_Step2.htm

# Of Hosts per SN-ID

128-2 = 126 (Invalid)
64–2 = 62
32–2 = 30
16-2 = 14
8–2 = 6
4-2 = 2
2–2 = 0 (Invalid)
0 (Invalid)

I have no idea why 126, 0 are invalide number of hosts. I appreciate any help.

Thanks
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Question by:aja101498
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Expert Comment

by:cf_root
ID: 9879070
I am not sure why they are listing 126 as invalid. Because technically with that subnet mask you have 126 available address. The 2 unusable addresses are for your broadcast and network. That is why the "2-2" is not valid. One is the network the other is the broadcast address therefore no hosts can reside on this mask. Please let me know if you have questions.
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Expert Comment

by:chicagoan
ID: 9879301
/25 is a valid mask
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Expert Comment

by:MaB
ID: 9880830
They are probably listing 126 as invalid because with a class C net subnetted into two 128 subnets with 126 possible hosts in each subnet that would mean that the first subnets network adress would be X.X.X.0 which is the same as the class C nets network address which is "no good".
Similar reason with the second subnet which will have X.X.X.128 as network adress which is "legal" but X.X.X.255 as broadcast adress which is the class C nets broadcast address thus also "no good".

You will always lose the first and last subnet as "no good" when subnetting.

See more extensive description in http://www.experts-exchange.com/Networking/Q_20816228.html#9874506
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Expert Comment

by:MaB
ID: 9880834
0 is invalid because you wont have need for a subnet that can't have ane hostes attached.
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Accepted Solution

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chicagoan earned 50 total points
ID: 9881954
This is a limitation of the windoze ip implementation, not because some other subnet class uses them as a network address or broadcast address. Using that logic a .252 mask would invalidate 124 addresses in a .255 network.
Supernetting always yields 0 and 255 host addresses, and these should be reserved in your allocation table for that reason.
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