Question Networking Exam Question

kevinm34232

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Apr 25, 2014
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I'm taking a Networking course and now I'm on the chapter dealing with IPs. This is one questions on the end of chapter exam, which really wasn't covered and I don't understand the answer:

Here is a configuration on a home network computing device:
IP Address: 172.16.2.1
Subnet Mask: 255.255.254.0
Default Gateway: 172.16.1.1
DNS: 192.168.1.1
Which of the following changes will allow the computing device to communicate properly?

a. Change the DNS to 172.16.1.100
b. Change the WINS to 172.17.4.136
c. Change the netmask to 255.255.255.0
d. Change the default gateway to 172.16.3.1


The correct answer was "d". Can someone explain to me why? Thank you!
 

kanewolf

Titan
Moderator
I'm taking a Networking course and now I'm on the chapter dealing with IPs. This is one questions on the end of chapter exam, which really wasn't covered and I don't understand the answer:

Here is a configuration on a home network computing device:
IP Address: 172.16.2.1
Subnet Mask: 255.255.254.0
Default Gateway: 172.16.1.1
DNS: 192.168.1.1
Which of the following changes will allow the computing device to communicate properly?

a. Change the DNS to 172.16.1.100
b. Change the WINS to 172.17.4.136
c. Change the netmask to 255.255.255.0
d. Change the default gateway to 172.16.3.1


The correct answer was "d". Can someone explain to me why? Thank you!
I believe it is because of the netmask and which IP addresses are in the allowable range. Put the base IP and netmask in to a web based subnet calculator.
 

kevinm34232

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Apr 25, 2014
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Hmm.. So if I do the bitwiseAND with that IP and mask I get 172.16.2.0 as the Network ID. I'm a little confused as the chapter went in to how to subnet a network into multiple subnets, but this is already done, and it didn't really discuss relationship to default gateway.
 

Math Geek

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the default gateway has to be in the subnet the device is on. the current .1.1 address is not in the same subnet as the device. but the .3.1 address from answer d is. therefore it would work as a default gateway for the device. it is obviously not the only possible gateway but from the answer choices it would work.

answer c would not work either since changing the mask still does not let .1.1 address to be in the subnet with the device since .2.0 would still be the subnet it is in.

from my experience taking a handful of cert tests lately, this is a VERY common type question. right away when you see any kind of odd mask, the first thing to do is compute the full ip range as most likely the answer will be related to the range available. you need to practice calculating this range over and over until it only takes a few seconds to do. i had many questions on the various tests that required me to compute the proper range to answer a question!
 

kevinm34232

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Apr 25, 2014
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That makes sense now. So are these ranges correct?

172.16.2.0 > 172.16.3.255
172.16.4.0 > 172.16.5.255
and so on?

I assumed that to subnet, 7 bits were borrowed in the 3rd octet, making the Delta "2". Is that right or is there a more obvious way that I missed?
 

Math Geek

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that works good enough. i've seen a number of ways to go about it and they all work. just get used to one and practice with it until you're real fast at it.

when you see such a question get that range computed and then look over the answers to be sure you're within it or outside it depending on the question. sometimes, the answer is something NOT in the range. but you have to read closely cause they are good at asking confusing ones to be sure you're paying attention.

lots of "won't connect" type questions end up with this type logic.
 

kevinm34232

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Apr 25, 2014
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They teach you how to subnet, but this confused me as it was already subnetted. I understand on using the bitwiseAND to get the network ID but the rest seemed clunky to me, what method do you use?
 

kevinm34232

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Apr 25, 2014
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Thanks for the info. It looks similar to 7 second subnetting I saw on youtube, which seems to be the fastest method and best for the exams. I was concerned about knowing all the ins and outs but it really just matters for the exam so speed is most important. In the real world afterwards, I guess you can do it any way you want!
 

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