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Identify a Subnet Mask number

Posted on 2015-01-27
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I recently took a test and I was unable to determing the Subnet Mask from a question on the test. I understand why from the information you would choose the 10.0.0.97 as the IP address and I understand why you would pick 10.0.0.110 as the default gateway but I completely do not undersstand how you would know the Subnet Mask of 255.255.255.240 from the info presented to you in the question.
SubnetMask.pdf
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Question by:James Coats
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mikebernhardt earned 300 total points
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the big clue is when they say that Host A uses a 28-bit subnet mask. If you look at the notes below the choices, they give you the math to understand why a 28 bit mask is going to be 255.255.255.240.

And here a couple of hints to make this sort of thing easier in the future: Remember that octets are always 256. so if you subtract 240 from 256, you have 16, which is the number of IP addresses in the subnet. So the network portion of the address 97 belongs to a network divisible by 16,  which will be 10.0.0.96 255.255.255.240.

It could of course be part of a larger network if the subnet mask was different. If it was a 27 bit network, the mask would be 255.255.255.224 and the network would still be 10.0.0.96, because 96 is divisible by 32. But go bigger and it will change.
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by:Trenton Knew
Trenton Knew earned 200 total points
ID: 40573513
the best  could tell you is that 255.255.255.240 (or a /28) subnet mask only leaves four bits available in the IP address space for the host.  And since "0000" would be the network address, and "1111" the broadcast address, you only have 14 assignable addresses in that space.  so 10.0.0.0/28 with a .97 means that network range for the subnet is 10.0.0.96 - 10.0.0.111  .96 is the network address, and .111 is the broadcast address. .97 is the first assignable IP and 110 is the last assignable IP.
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by:mikebernhardt
mikebernhardt earned 300 total points
ID: 40573533
You will also find it useful to just memorize your subnet tables: 32 bits is 255; 30 bits is 252; 29 bits is 248, etc.

Get into much larger networks and you might have 10.4.0.0/14, which would be a subnet mask of 255.252.0.0. That would include the range of 10.4.0.0-10.7.255.255.
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by:James Coats
ID: 40573564
Damn you guys are WAY better than my teacher who I now don't think she really knows what she is talking about. This is fairly simple now given the way you have explained it to me. I was so confused by my teacher that I think when I have questions I'll just post here as I have to take time to see her and such.

I think the public schools and community colleges will go the way of the Post Office. This is much faster and better. Thank you very much.
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by:mikebernhardt
ID: 40573604
Thanks! Keep in mind though that EE has rules against asking for help with assignments. In this case you asking for help to understand a completed one, so it was OK.

Also, there is the "right way" to understand it but that's not always the easy way :-)
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