Basic Subnetting question

Unanswered Question
May 12th, 2008

Newbie here just getting my head around subnetting and have a question.

When using the subnet mask to identify the network portion and the host portion of the subnet mask and then working out how many subnets and useable host addresses that would give us, we use the 2 to the power x -2 (where x is the number of bits used for the host or network portion).

I fully understand the 2n-2 for the host addresses as the -2 represents the network address and the broadcast address, however I don't understand why we need to do -2 for the amount of subnets available.

An example question I've found on sample CCNA test papers and CBT courses is when a class C address is given with a 28 bit mask and you are asked to work out how many subnets and how many usable host addresses you would get. You are given multiple choice answers - one of the answers is usually 16 subnets with 14 usable host addresses and one of the answers is 14 subnets with 14 usable host addresses (plus 2 other irrelevant answers)

I'm happy with the 14 usable host addresses using the 2n-2 formula, but the correct answer according to the papers/course is 14 subnets - again this makes sense using the 2n-2 but I am just not sure why we should be using the -2 bit of the formula when calculating subnets? Why isn't it 16 subnets with 14 usable host addresses?

Will probably be very obvious and I've missed the point completely.

I have this problem too.
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Jon Marshall Tue, 05/13/2008 - 00:33

Paul

An example might help.

192.168.5.0 255.255.255.0

Subnetted using a 28 bit mask ie. 255.255.255.240

192.168.5.0

16

32

48

64

80

96

112

128

144

160

176

192

208

224

192.168.5.240

So it is 16 useable subnets with 14 hosts per subnet. The 2n - 2 for subnets is quite old now. This used to be relevant when you could not use the first and last subnets ie.

192.168.5.0/28 & 192.168.5.240/28 because devices were not capable of distinguising between 192.168.5.0/24 and 192.168.5.0/28.

This is what "ip subnet-zero" does. It allows you to use the first subnet ie. 192.168.1.0/28 and is now the default on Cisco routers/L3 switches.

Jon

Paul_Edwards Tue, 05/13/2008 - 02:59

Many thanks Jon - the course/papers are a bit old. Does seem a bit pointless to use the -2 for subnets as surely you are wasting 2 subnets using the -2.

Thanks for clearing it up :)

rob.huffman Mon, 05/26/2008 - 05:09

Hi Jon,

I thought that this was a pretty great answer, surely worthy of at least my 5 points!

Take care,

Rob

Jon Marshall Tue, 05/27/2008 - 05:35

Rob

Many thanks. I was a bit puzzled as to why it originally got a "not helpful" :-)

Jon

Paul_Edwards Wed, 06/04/2008 - 00:14

Not sure where you got the "not helpful" bit from Jon - I've not seen anywhere to select how helpful the post was or would have selected "very helpful" :)

Richard Burts Wed, 06/04/2008 - 10:18

Paul

In the box for "rate this post" is a drop down of values to use to rate the post. A 1 is labeled "not helpful". A "very helpful" is a 4 and "extremely helpful" is a 5.

And if you initiated the thread there is also a box that you can check to indicate that a response did solve your problem or question.

Using the rating system is helpful because it can help people identify threads where there was really helpful information.

HTH

Rick

Jon Marshall Wed, 06/04/2008 - 10:42

Paul

As per Rick's post 1 is not helpful and the post was initially rated a 1.

However i did not assume it was you that rated it at all as you posted a thanks for the information i provided so my comment was not directed at you.

Unfortunately some people just like marking 1 regardless of how helpful or not the post was.

Jon

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