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simple subnetting question

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

cf_root

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.

chicagoan

/25 is a valid mask

MaB

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 https://www.experts-exchange.com/questions/20816228/Subnetting.html#9874506

MaB

0 is invalid because you wont have need for a subnet that can't have ane hostes attached.

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chicagoan

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