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Subnet Question 10.25.66.154/23

Subnetting

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#1 Blaize

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Posted 02 August 2013 - 04:01 AM

Decimal IP Address  Address   Number of Subnets    Number of      Number of 

                                      Class    and Host Bits              Subnets           hosts (2X – 2)

10.25.66.154/23              A        11111111.11111111.111111100.00000000 note because this is a class A address count the number of bits for 255.0.0.0.  15 subnets 9 host bits 

2X – 2 2 9 – 2  =              512     256   128   64   32 16   8    4     2   1   = 512 – 2 = 510, number of subnets  2X = 2 15 =  32768

 

 

My question to you all is, why dont we count the First octet?

I understand where the hosts is comming from, but i thought that for the network, wed count all of the "1"s.

 

Which would be 2/\ 23rd no?

I seem to have a hard time with class a addresses that surpass the first octet.


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#2 Darby Weaver

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Posted 02 August 2013 - 01:50 PM

111 111 10. 0000 0000

Ok. The question you have in regards to not using the first subnet is an old rule. We did not use the first subnet or the last subnet until "ip subnet zero" was introduced and since become the default for the Cisco IOS.

Look up ip subnet zero for a more detailed explanation of the concept.

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#3 Blaize

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Posted 03 August 2013 - 03:25 AM

Theres a typo in the 3rd octet.

But the reason we dont use the first octet is the ip subnet zero, correct?


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#4 Darby Weaver

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Posted 04 August 2013 - 06:11 AM

The ip subnet zero command allows us to use the first and last sinners that previously were unusable.

The "no" ip subnet zero command would make the first and last sinners unusable again.

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