Subnetting

[waseem abdo]

An office has 8 floors with approximately 30-40 users per floor. What command must be configured on the router Switched Virtual Interface to use address space efficiently?

• A. ip address 192.168.0.0 255.255.255.128
• B. ip address 192.168.0.0 255.255.0.0
• C. ip address 192.168.0.0 255.255.255.224
• D. ip address 192.168.0.0 255.255.254.0
  Now here is someone's explaination:
  

  We cannot use the subnet mask of 255.255.254.0 (/23) because it is smaller than the default subnet mask of class C (/24). If we use /23 subnet mask, in fact we are summarizing (supernet) it -> Answer "ip address 192.168.0.0 255.255.254.0" is not correct.

  With the subnet mask of 255.255.255.128, we have 21 = 2 subnets only, not enough for 8 floors -> Answer "ip address 192.168.0.0 255.255.255.128" is not correct.

  We need 40 users per subnet so our subnet must support at least 64 (=26) hosts (in fact 62 hosts). So the last octet of subnet mask should be 1100 0000 (with 6 bits “0”) = 192 -> The suitable subnet mask is 255.255.255.192. But this subnet mask only provides 22 = 4 subnets, which is not enough for 8 floors.

  For 8 floors we need 1110 0000 (23 = for the last octet of subnet mask so the subnet mask should be 255.255.255.224.

  But with the subnet mask of 255.255.255.224, the host addresses per subnet is 25 - 2 = 30 so we cannot fulfill the host addresses for 40 users. We can only provide IPs for 30 users. This question said "approximately 30-40 users per floor" so "30 users per floor" is acceptable.
  For me I believe this is wrong and the answer is D.

• Another question: I can use 255.255.0.0 with a class C address, right?

[Georg Pauwen]

Hello,

tricky question. Answer C gives you a Class C with 8 subnets and 30 users per subnet. That is probably what they are looking for. If Class C is a requirement, you need to have at least 24 bits, so anything less would not be a Class C anymore.

[Gopinath_Pigili]

I think that B is the correct answer. 2^9 = 512 , 8 floors*40 users/floor = 320 --> more addresses that are needed
At option A are only 126 usable IP addresses, so it's not enough....

In reality...as Georg Pauwen mentioned...class C shouldn't be less 24-bits....only for examination point of view B is the answer.

Best regards
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[waseem abdo]

You mean answer D is the correct answer not B because answer B gives you 65534 valid IP addresses.

[Gopinath_Pigili]

Yes...you are right...

sorry for the typo error...for me I think Answer: D  

192.168.0.0 255.255.254.0

Best regards
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[Joseph W. Doherty]

Question is unclear!

Is there but one SVI for all 8 floors or one SVI per floor?

If only one SVI (which I believe is question's intent), we need to support up to 320 hosts (8 x 40).  For that we need a /23, i.e. answer D.

You mention 192.168.x.x being a Class C IP.  Yes, that true, but only a concern if using a Classful dynamic routing protocol like original RIP.  An interface will work just fine using answer D.

[waseem abdo]

Yes I believe it is just one SVI for all 8 floors but the question was really unclear about it.

Answer D is correct if it is for only one SVI and answer C is correct if it is for one SVI per floor.

[Joseph W. Doherty]

". . . answer C is correct if it is for one SVI per floor."

No, it's not, as a /27 only supports 30 hosts and we may have up to 40 users.

[waseem abdo]

But answer B has 65534 valid IP addresses which is too much needed for the requirement and that is why I say answer is D.

[Joseph W. Doherty]

Yup, especially since ". . . Switched Virtual Interface to use address space efficiently. . .".

[Joseph W. Doherty]

Another question: I can use 255.255.0.0 with a class C address, right?

Depends how you intend to use.  The Class C address block is a /3.

[waseem abdo]

So why the explaination said we cannot use subnet mask smaller than 24 (255.255.255.0) if it is just fine to use it in real devices (as I already know)?

[Joseph W. Doherty]

Possibly because "someone's explanation" is incorrect.

"Someone" doesn't appear to fully understand Classful addressing, subnetting and/or supernetting.