code:

Here's today's question.

Given the network 172.16.31.0, you must create 6 subnets to 
support 24 users each.  How many are available for future growth?


Please remember, don't reply to this thread with the answer, 
so that everyone can have the opportunity to try it for 
themselves, and the answer will be posted tomorrow.



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Now for the answer to the 5/6/03 question.

Which of the following statements are true?

A. 172.16.168.0/27 is a valid host address
B. 172.16.168.255/23 is a valid host address
C. 172.16.168.0/23 is not a valid host address
D. 172.16.168.255/26 is a valid host address 


Answer:  Both B and C are correct.  172.16.168.1-172.16.169.254 are 
valid hosts on the /23 network. 

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Should we assume /24 because of the .31 in the third octet?

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Who proof read these? They should be flogged!!!

Oh, wait, er, uh, never mind...

Yes, it should have been 172.16.31.0/24 for the provided network.

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So I got C right, but I don't understand B...

B. 172.16.168.255/23 is a valid host address

I thought it means if 172.16.168.255 is a valid host address.... I thought 255 is for boardcast...then its not a host address

Please explain.

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The thing to remember is that "255" is the broadcast address ONLY if it makes the host address all 1s, as in a /24 network.

Look at the bits:

A /23 means use the left 23 bits for network, and the right 9 bits for hosts.

code:


172.16.168.0/23  <network ID (all 0's)

172. 16.1010 1000.0000 0000
255.255.1111 1110.0000 0000 
     --| SN    |---------- <--subnet
               |  Hosts   |  

172.16.168.255/23

172. 16.1010 1000.1111 1111
255.255.1111 1110.0000 0000
     --| SN    |---------- <--subnet
               |  Hosts   | 

The host bits for this network are 011111111, not all 1's.  

The /23 mask provides 510 host addresses, and the valid IP hosts for this network are 172.16.168.1 through 172.16.169.254.

The secret - when in doubt, convert the decimal values to binary and map out the range of valid hosts, as well as the range of valid networks.

HTH...

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WOW, thank god you are posting these great questions. I don't think the Sybex book teaches it.... or maybe I missed it.

Very tricky, tricky

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Thanks for the kind words!

I know how confusing the topic can be, and the cert exams take full advantage by wording the questions very creatively. If I can help you practice with some tricky questions, then the easy ones will be a snap!

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Giving subnetting questions is a great idea!!

On a CCNA exam, one is most likely to see a classful address being used, ie. 172.16.0.0/16. Thisquestion would most likely be asked using a class C address -- 192.168.1.0/24.

__________________
Cisco Teacher

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quote:

---

Here's today's question.

Given the network 172.16.31.0, you must create 6 subnets to
support 24 users each. How many are available for future growth?

---

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Hi, DXU76.

To find the answer to a question, look for the thread with the next date, as I include the answer with the next question.

Here's the link for this question:
http://www.examnotes.net/article1009458.html

If you can't find the thread, you can always PM me, and I'll help you out.

Again, I call it [bold]"daily"[/bold] because while I will try to get it out every day, I can't guarantee I will be able to. Thanks!

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