70-291 subneting question

brunobliss

Hey guys, i will be taking the exam in a couple of days and just found some exercices to practice. While i done well in most, there is this question that i have some doubts with:

You need to ensure Server1 can comunicate with computers in the other offices.
which subnet mask should you assign to Server1?

A. 255.0.0.0
B. 255.255.255.192
C. 255.255.255.128
D. 255.255.255.0



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CORRECT ANSWER IS B ??


My answer was D (/24). Now i'm pretty sure that using /24 would place server1 and the router on the same subnet (sicne the host range from 1-254), so , what am i thinking wrong?

Devilsbane

I'm not a subnetting genius (and it's been quite some time), but here is my take on the question.

A: This is obviously wrong because it would place every address up there on the same network.

B: I like to write these as 255.255.255.11000000. This will give you 4 subnets. 0-63, 64-127, 128-191, and 192-255. With this, the .129 and .222 in the top branch would fall in different networks.

C: 255.255.255.10000000 This will give you two subnets with range from 10.10.10.0 to 127 and then your second range from 128 to 255. As you can see, the bottom branch would be the first subnet and the top branch would be the second subnet.

If you used a standard class C address then noth branch offices would be on the same network. You are correct that Server1 and that top router would be on the same network, but so would the bottom router.

Does that help?

brunobliss

hmm interesting, thank you for sharing your point of view, this is actually a very tricky question, but i get it now:

Using /24 would put top and bottom branch computers in the same subnet.

But using /26 will divide the number of hosts over two subnets 1-127 and 129 - 254.

Since this second subnet ( 10.10.10.128 ) is where the top branch server and router are in, they will no longer be in the same as the 10.10.10.0.

Still, this is one of those questions that most people are likely to fail ... a true brain crunching exercise.

Devilsbane

brunobliss wrote: »

Still, this is one of those questions that most people are likely to fail ... a true brain crunching exercise.

Thats their goal. Someone is making a fat paycheck somewhere for making you fail. To me, this is the difference between a CompTIA exam and a Microsoft exam. Network+ might ask something like which of these addresses is not in the subnet while Microsoft expects you to sit down and do a little bit of work.

I wouldn't fear it, subnetting is a good thing to know (at least while IPv4 hangs on) but it isn't criticial to this exam. You won't see it many times and they even give you the windows calculator to help you out. I just gave someone advice on their Security+ exam, and it applies here as well. Read the question, read all of the answers, select the best one, and then go back to the question and make sure it still makes sense. If you are not 100% on something, answer it as best as you can and then mark it to come back to (if you have time which you should). Sometimes this second look gives you a different view and you can spot an error that you made the first time. But don't change it unless you are positive your first answer is wrong. Many people have talked themselves out of a point.

brunobliss

wait, in the meanwhile, i just realized i was wrong from the start:

10.10.10.222 and 10.10.10.129 won't fall in the same subnet if the subnetmask is /26 because the range of addresses goes from 10.10.10.129 to 10.10.10.190 for the ( 10.10.10.128 subnet) ...


Means, using /26 still makes no sense as Server1 would not be able to communicate with top router unless there was another gateway for that subnet.

I'm starting to believe none of the answers is right, but i would still choose /24 then ... Heck! i'm putting this to the test in packet tracer !!! brb

Devilsbane

That is correct, /26 is incorrect because the server and the router in the top branch wouldn't work. (Router would be in subnet C while the server is in subnet D)

/25 should be the correct answer, which is answer C

Subnet A: 10.10.10.0
....Address range 10.10.10.0 to 10.10.10.127 (Bottom Branch)
Subnet B: 10.10.10.128
....Address range 10.10.10.128 to 10.10.10.255 (Top Branch)


The /26 would look something like this...
Subnet A: 10.10.10.0
....Address range 10.10.10.0 to 10.10.10.63
Subnet B: 10.10.10.64
....Address range 10.10.10.64 to 10.10.10.127
Subnet C: 10.10.10.128
....Address range 10.10.10.128 to 10.10.10.191
Subnet 10.10.10.192
....Address range 10.10.10.192 to 10.10.10.255

brunobliss

Yes i agree and i hope people in the future stumble upon this and conclude the same as we did.

Although i failed this question by choosing /24, at least now i can come to a sense why /25 is the only answer that truly makes sense, and that the answer provided by the solution is dead wrong!

Case closed

Devilsbane

Mistakes happen a lot, this is why using multiple sources and asking questions is valuable.

I'm thinking they either screwed up one of the addresses (129 instead of 229?) or they just marked the wrong one as correct.

someeh

brunobliss wrote: »

Hey guys, i will be taking the exam in a couple of days and just found some exercices to practice. While i done well in most, there is this question that i have some doubts with:

You need to ensure Server1 can comunicate with computers in the other offices.
which subnet mask should you assign to Server1?

A. 255.0.0.0
B. 255.255.255.192
C. 255.255.255.128
D. 255.255.255.0



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CORRECT ANSWER IS B ??


My answer was D (/24). Now i'm pretty sure that using /24 would place server1 and the router on the same subnet (sicne the host range from 1-254), so , what am i thinking wrong?

funny thing I saw this question last night!!! I took my exam today and saw the same question lmao.... didn't help the fact I failed it.

brunobliss

sorry to read that you failed it I passed the exam on friday with 880 marks.

Still, i had major doubts with about 10 questions and i'm pretty sure some questions had two correct answers, i've read those questions and answers over and over till the time was nearly out and at least one of them had vital information missing:

you had to configure a server's NIC so it could communicate with other computers in the same network, easy, but should you also apply the gateway? None of the other computers had it, and the question did not specify that requirement, so i did not configure it ... still, the gateway computer was there, for what specific reason?

i should had left a comment on that one, its like someone's expecting you to choose a correct one giving you only this info:

you need to make sure the dhcp is leasing addresses, what should you do?

. turn on the computer
. authorize the server
. activate the scope

This was not a real question but believe me i've analyzed some of the questions and sometimes you just needed more info to know for sure you were providing the correct answer.

knownhero

I had that question on my exam.