Subnetting - Reason for maximum number of subnets

His question was not "how do I subnet". If it were, then yes it is duplicate an answered in the linked thread.

His question was where is the instructor getting that he is borrowing 3 bits:

I didn't really understand why he said he'll be using 3 bits

That question is not directly answered by the linked thread, therefore this is not a duplicate. The other thread is related, and should be linked and referred to as additional reading and context. But not used as a reason to close the thread.

The only reference to borrowing bits in the linked thread does not also include the borrowing of bits from a subnet's default Class a/b/c designation. Again, the linked thread does not answer the question.


That question is answered in the Calculating subnets section in the answer by Mike Penningtion. The answer discusses borrowing bits from the host bits, and it gives the formula for calculating the necessary number of host bits. That answer tells the OP how to do what is being asked.

Classful networking is dead, killed in 1993 by VLSM and CIDR (see RFCs 1518 and 1519), and the question of network classes was never raised in the original question.

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    No. It is not. The section describes how to subnet a major network into smaller minor networks. It does not answer where "3 bits" comes from with the claim that borrows three bits. It does not answer the question of where the borrowed bits come from, it answers the question of what to do with the borrowed bits -- there is a difference. – Eddie May 23 '16 at 17:46
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    I believe it does. It says, "...as a general rule you build subnets by reallocating some of the host bits as network bits for each subnet." The example has a picture showing this. That is exactly what the question is asking about. No, it didn't specifically answer why three bits, but it gives enough information to figure that out since the example is borrowing three bits. – Ron Maupin Mod May 23 '16 at 18:00
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    No, it didn't specifically answer why three bits then it clearly isn't a duplicate, is it? It doesn't give enough information to figure it out because there is no mention of the (antiquated) IP Address Classes, which must be mentioned to answer why a /27 borrows 3 bits. Why was the starting point /24? Why not /20 or /21 and borrowing 7 or 6 bits ,respectively. – Eddie May 23 '16 at 18:24
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    The classes have nothing to do with it. In the question, it was given that the network uses 24 bits: "He said 24 bits are for network address and 8 for the host (understood)". That really has noting to do with network classes, and the mention of network classes only confuses things. Following Mike's answer explains why three bits are used for the subnet, even though Mike's answer uses 21 bits for the network. Classes don't play any part in the question or answer. – Ron Maupin Mod May 23 '16 at 18:29
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    Then a potential answer could have been "because he said it in point 1", which again is not something answered in Mike P's thread. Once again, there is a difference between a related question and a duplicate question. – Eddie May 23 '16 at 18:38
  • If the first point said that it was 21 network bits and 11 host bits, would that lead you to explain network classes? That would have exactly matched Mike's answer. Just because the number of bits leads to octet boundaries, you assumed network classes, which have nothing to do with the question. The answer to the question, as written, is in Mike's answer. – Ron Maupin Mod May 23 '16 at 18:41
  • The question was obviously a reference to classes... but don't take my word for it, take the original poster marking my "class based" answer the right answer. The Network Education field constantly makes (useless) references to Classes, and it causes the same confusion, I see it all the time and recognized it in the original poster. Had he never been introduced to classes, then there never would have been an ambiguity. All of his subnetting questions would be taking a defined subnet size to another defined subnet size, and there would be no question of "why X amount of bits were borrowed". – Eddie May 23 '16 at 19:13
  • So, you have a question in which the number of network and host bits are specifically given, and there is no mention of network classes. You think the question has an obvious reference to network classes? I just don't see how you connect those dots. Given a /24 network, from whichever network class, you must still borrow bits from the host portion to subnet it. There is no need to introduce network classes into the question or answer. If the network was (Class A), you still need to borrow three bits from the host portion of the network. – Ron Maupin Mod May 23 '16 at 19:28
  • Are you arguing why the answer had to do with classes or why the question was duplicate or not. – Eddie May 23 '16 at 19:49
  • I'm saying that Mike's answer answer's the question, and that it has nothing to do with network classes. The answer clearly discusses borrowing bits fro the host portion. The addition of network classes has nothing to do with the question. The instruction video may have never mentioned nor intended network classes, and the OP may now be led down the wrong path. – Ron Maupin Mod May 23 '16 at 19:56
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    And if the instruction video did refer to classes, linking to a non-duplicate thread and claiming it as duplicate could have lead the OP down the wrong path. So at its core, you don't know what the intent was, so how could you possibly determine the question should be dismissed. This comes back to erring on the side of being helpful, or erring on the side of being dismissive. Which do you think builds a better community? Which do you think has more of a chance of bringing that user back? – Eddie May 24 '16 at 0:32
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    Mike's answer did answer this question, and adding things which were not referenced, nor necessary to answer the question, in the question is a complete non sequitur. This site is meant as an archive for people to do research, and mentioning network classes where none are needed can be very confusing. This is not a Network Learning site, but, as it says, "Network Engineering Stack Exchange is for asking questions about professionally managed networks in a business environment." I think you should petition SE to open a Network Learning site, rather than trying to turn this site into one. – Ron Maupin Mod May 24 '16 at 0:41

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