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IP Grouping

 
 
RogueIT
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      02-25-2009
While answering a subnetting question I got this one wrong. Can someone
explain the answer to me?

TIA
Rogue.
******************************************
Question:
Suppose that your network has an Internet Assignted Numbers Authority
(IANA)-assigned Class C CIDR block of eight addresses. Which subnet mask
groups these IP address into a single subnet.

255.255.240.0
255.255.248.0
255.255.255.0
255.255.255.248
 
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Lawrence Garvin \(MVP\)
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      02-25-2009
"RogueIT" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> While answering a subnetting question I got this one wrong. Can someone
> explain the answer to me?
>
> TIA
> Rogue.
> ******************************************
> Question:
> Suppose that your network has an Internet Assignted Numbers Authority
> (IANA)-assigned Class C CIDR block of eight addresses. Which subnet mask
> groups these IP address into a single subnet.
>
> 255.255.240.0
> 255.255.248.0
> 255.255.255.0
> 255.255.255.248



The question is, arguably, poorly written.

The apparently obvious answer is to the question: What subnet provides for
eight hosts, and that answer is 255.255.255.248.

But I suspect the intent of this question is to ask: Suppose that your
organization has an Internet Assigned Numbers Authority (IANA)-assigned
block of eight Class C CIDR networks. Which subnet mask groups these eight
networks into a single supernet.

And the answer to *that* question is: 255.255.248.0


--
Lawrence Garvin, M.S., MCITP(x2), MCTS(x5), MCP(x7), MCBMSP
Principal/CTO, Onsite Technology Solutions, Houston, Texas
Microsoft MVP - Software Distribution (2005-2009)

MS WSUS Website: http://www.microsoft.com/wsus
My Websites: http://www.onsitechsolutions.com;
http://wsusinfo.onsitechsolutions.com
My MVP Profile: http://mvp.support.microsoft.com/pro...awrence.Garvin

 
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RogueIT
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      02-25-2009
> And the answer to *that* question is: 255.255.248.0

That is the correct answer but how did you come to get 248.0?

 
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Lawrence Garvin \(MVP\)
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      02-25-2009
"RogueIT" <(E-Mail Removed)> wrote in message
news(E-Mail Removed)...
>> And the answer to *that* question is: 255.255.248.0

>
> That is the correct answer but how did you come to get 248.0?


Consider this block of contiguous "Class C CIDR" networks:

192.168.64.0
192.168.65.0
192.168.66.0
192.168.67.0
192.168.68.0
192.168.69.0
192.168.70.0
192.168.71.0

any one of those networks would have a subnet mask of 255.255.255.0 -- a
standard Class C network.

increasing the fourth octet pushes us into subnets-of-one-Class-C territory,
and that's not where we want to go, we need to make the adjustment the other
direct, in the third octet, to combine individual networks into a
supernetwork.

Using simple brute force methodology and working backwards:

255.255.255.0 = 1 Class C network (e.g. 192.168.64.0 with 256-2 hosts in
the network)
255.255.254.0 = 2 Class C networks (192.168.64.0 and 192.168.65.0, with
512-2 hosts in the supernet)
255.255.252.0 = 4 Class C networks (192.168.64.0 thru 192.168.67.0, with
1024-2 hosts in the supernet)
255.255.248.0 = 8 Class C networks (192.168.64.0 thru 192.168.71.0, with
2048-2 hosts in the supernet)

Or, you can also use some basic arithmetic...

Just as 256 - 8 = subnet mask of 248 for 8 *hosts* in a single Class C
network,
so does 256 - 8 = subnet mask of 248 one octet up, to get 8 *networks*.




--
Lawrence Garvin, M.S., MCITP(x2), MCTS(x5), MCP(x7), MCBMSP
Principal/CTO, Onsite Technology Solutions, Houston, Texas
Microsoft MVP - Software Distribution (2005-2009)

MS WSUS Website: http://www.microsoft.com/wsus
My Websites: http://www.onsitechsolutions.com;
http://wsusinfo.onsitechsolutions.com
My MVP Profile: http://mvp.support.microsoft.com/pro...awrence.Garvin

 
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