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We discuss that topic but we have too different interpretations. That is why I'd like to know how to resolve this issue. What is the truth? Can I define max 64 subnets or max 4?

Writing down an opinion will not suffice, sorry. We already have 2. We need to scientifically show (for example with official network standards) which is true.

womble
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erikbstack
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3 Answers3

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A /26 network has 64 addresses. Since the definition of a subnet is that it contains at least 2 addresses, the technically correct answer is 32. However, for practical implementations the actual answer is more like 16, since in order to be really useful, 2 addresses in each subnet (the network address and the broadcast address) should not be used for other purposes, and therefore a useful subnet comprises 4 addresses.

wolfgangsz
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  • this is one oppinion. Can u prove that with sources please? Other people think it is different. To name it they think the x/26 means that there are 2 bits specifically defined to be used for subnets, and thus there are only FOUR subnets possible with 64 adresses (or 62, doesn't matter for this discussion) each. – erikbstack Jul 13 '11 at 12:21
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    is this a homework question? – SpacemanSpiff Jul 13 '11 at 12:24
  • My understanding of a /26 network is that the 26 highest bits of any address pertaining to the subnet are fixed (as per the standard definition of classless networking, CIDR), which then leaves the remaining 6 bits for hosts or subnets within the network. – wolfgangsz Jul 13 '11 at 12:26
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    erikb, wolfgangsz' response is not an opinion, it's simple math. See massimo's reply for a textbook-quality explanation. – Jodie C Jul 13 '11 at 12:28
  • @SpacemanSpiff It is preparation for exams. We discuss what we learned in the class without the teacher, so we can't ask him, to help us interprete his script. – erikbstack Jul 13 '11 at 12:31
  • @Jodie C: It makes sense, doesn't mean it is true. The other oppinion also makes sense. That is the thing about standards. Sometimes many oppinions are good, but u need to choose one. We want to know what is correct concerning TCP/IP. The solution to the question is prove (source) how X and slash-number is defined, concerning creating subnets. – erikbstack Jul 13 '11 at 12:32
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    Please read http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing for details. This article contains numerous links to the various RFC documents going to extreme detail. If you get taught anything different, then your teachers are out of sync with reality. – wolfgangsz Jul 13 '11 at 12:37
  • @wolfgangsz yes that is the answer that I'm looking for I think. So the X has no special meaning? If it doesn't I can prove to my classmades that what u say is true. – erikbstack Jul 13 '11 at 12:54
  • The "X" is any permissible value that identifies one of the 4 /26 networks within 192.168.123.0/24, i.e. 0, 64, 128 or 192. Other than that it has no impact on the answer to your question. – wolfgangsz Jul 13 '11 at 12:59
  • You should ask if they want some kind of textbook answer or a practical answer. – SpacemanSpiff Jul 13 '11 at 13:36
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You can create:

  • 1 subnet containing 64 addresses (/26)
  • 2 subnets containing 32 addresses (/27)
  • 4 subnets containing 16 addresses (/28)
  • 8 subnets containing 8 addresses (/29)
  • 16 subnets containing 4 addresses (/30)
  • 32 subnets containing 2 addresses (/31)
  • 64 subnets containing 1 address (/32)

Given that a IP subnet always has two reserved addresses, the lowest one (network) and the highest one (broadcast), and so needs at least two more addresses to be actually usable, the lowest number of addresses you can have in a "real" subnet is 4.

Of course, you can also mix them, because every subnet can be re-subnetted; so you could f.e. have a /27 subnet using up the first 32 addresses, and two /28 subnets using the remaning 32 ones, dividing them in two blocks of 16 addresses. In any case, by using more smaller subnets, you are going to waste more addresses (due to each subnet requiring two reserved ones).

As usual, you can find everything you need to know (and much more) here.

Community
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Massimo
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  • I refer u to my comment on the other post. – erikbstack Jul 13 '11 at 12:26
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    See edit and link. This is *classless* IP addressing, so you can subnet *any* network as much as you like, and then it can be re-subnetted again, and again, and again, until you use up all addresses. – Massimo Jul 13 '11 at 12:29
  • what is specific about classless subnetting? maybe that is the problem in my group. – erikbstack Jul 13 '11 at 12:44
  • Please, follow the link I provided. This subject has been discussed *in depth*. – Massimo Jul 13 '11 at 12:45
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    @Massimo, PtP links may use /31 on OSes that support it. See [RFC 3021](http://tools.ietf.org/html/rfc3021) for details. (Almost every major OS and networking device supports /31 except Windows) – Chris S Jul 13 '11 at 13:07
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    @Chris, I know this but I preferred not to introduce too many details and edge cases in my answer ;-) Thanks for pointing it out anyway :-) – Massimo Jul 13 '11 at 14:37
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Given a /24 subnet mask and allocating an additional 2 bits for subnetting to create a /26 subnet mask means that you've allocated 2 bits (from the host portion) from which to create 4 subnets of the original /24 network. But, that doesn't mean that you can't subnet the /26 subnet further... you can. It seems that you're assumption (or proposition) is that you can't subnet any further, and for the purpose of the debate you're trying to settle that may be true if the debate asks the question "How many subnets can I create with a /26 subnet mask", without considering the possibility of further subnetting.

The /24 is the original network, it is not a subnet (although it could be a subnet of a larger network). The /26 mask allows you to create 4 subnets from the original /24 network.

joeqwerty
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  • The last part is what my classmates say. Btw the maybe weird thing is that I actually have the oppinion that u can create 32 subnets (minus the bit for network id and broadcast id). I want to prove successfully that it is not my stupid oppinion (I'm the only one who thinks that) but the truth. So, ur last sentence actually sounds like what the other people say. Can u explain further why u say 4 now and not 32? – erikbstack Jul 13 '11 at 13:05