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Today during a free online CCENT exam I was given the following question:

Which 2 statements best describe IP Address 10.16.3.65/23?

The 2 answers were:

The Broadcast Address of the subnet is 10.16.3.255
The Lowest Host Address in the subnet is 10.16.2.1/23

My questions are, how is this question solved methodically and whats happening to the 3rd octet? It seems to be changing from .3 to .2 when there is only 1 bit in the 3rd octet to play with (NNNNNNNH).

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  • It doesn't matter if experienced network engineers can relate to studying subnetting or not - these types of questions are specifically called out as off-topic for the site. There are literally hundreds of subnetting tutorials that can be found via a simple google search. Dec 24 '14 at 16:29
  • The question falls under the categories of "Lab Scenarios" and "Network Protocols' Design or Theory," both of which are explicitly on-topic for this site. It's a well worded question and will help others.
    – user6423
    Dec 24 '14 at 16:40
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    @Ben no, it doesn't. See the section on "multiple choice questions." The phrase "during a free online CCENT exam" should have given it away for you. Even if he phrased it differently he would just be adding to the dozen plus subnetting questions that already exist on NESE. Dec 24 '14 at 19:21
  • Referencing a certification exam does not change the fact that the question cleanly falls into the allowed topics.
    – user6423
    Dec 24 '14 at 19:33
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Think of it like this.

Consider the third octet of the /23 mask: 11111110 What does that rightmost 1 represent? It represents the 2s place, so the interval between subnets is 2. Some example subnets would be:

10.16.0.0/23 10.16.2.0/23 10.16.4.0/23 10.16.6.0/23

And so on. Now, the particular IP address in question, 10.16.3.65, falls into the 10.16.2.0/23 subnet. Hence, you just need to look at the subnet itself to determine the answers to your questions.

The broadcast address is the highest address before the next highest subnet. In this case, 10.16.3.255. The lowest host address or the first usable IP is one more than the subnet address. Since the subnet address is 10.16.2.0 the lowest host address is 10.16.2.1.

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  • Beautifully explained Ben. I'm kind of kicking myself right now.
    – Ben
    Dec 24 '14 at 16:28
  • Glad I could help!
    – user6423
    Dec 24 '14 at 16:51
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Don't look on octets in this questions. IPv4 is 32 bits. /23 mean first 23 bits is network address, other bits is host address.

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