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subnetting

Posted on 2014-10-03
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I'm studying for the Network + test, and for subnetting, I don't have a problem subnetting class C networks, but I'm confused on class B.
the method the book I'm using is to ask 5 questions
how many subnets - the formula is 2 to the X power - x = number of masked bits - no problems there
how many hosts per subnet - formula is 2 to the Y power minus 2    y = number of host per subnet
what art the valid subnets formula is  265 - subnet mask = block size aka increment number   -  now this is what throws me,
so, for example, if i have a class B network and the mask is 255.255.240.0 (/20) i would use the formula
256-240 = 16 so the block size or increment number is 16. in the book, he makes a chart like this

subnet        0.0         16.0              32.0          48.0
first host     0.1         16.1              32.1          48.1
last host     15.254   31.254         47.254      63.254
broadcast   15.255   31.255        47.255       63.255

so does this mean that in the first column for instance. if the address is 172.16.X.Y the first host address is 172.16.0.1 and the last address in subnet 0.0 is 172.16.15.254? which means there are LOTS of hosts?
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Question by:JeffBeall
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Ken Boone earned 167 total points
ID: 40360147
Yes that is exactly what it means... It also measn that 172.16.1.0 and 172.16.2.0 and 3.0, etc.. are all host addresses inside that large block.

Now in the real world you would only carve a class B this large to hand out to sub-organizations where it will be chopped up further.  You would never want a flat network that large.

But you got it!
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by:Fred Marshall
Fred Marshall earned 167 total points
ID: 40360153
Forget about "classes" for a moment at least.
I don't know why people continue to introduce this archaic term.  Lazy I guess.

But, to answer your question.  Yes.  That's it.  You have interpreted the table correctly.
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by:Jody Lemoine
Jody Lemoine earned 166 total points
ID: 40360154
You've got it exactly. The example you gave gives you 4094 hosts per subnet and 16 equally-sized subnets off of your /16.
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by:JeffBeall
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ok, thank you.
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