Subnetting - Number of Subnets Available

superman859superman859 Member Posts: 55 ■■□□□□□□□□
I recently started studying for the Net+ exam and am reading through Mike Meyers 'All In One Net+' book. I'm in the section on subnetting but came across something that didn't sit well with me.

He says the number of subnets given a network ID would be 2^n - 2 where n is the number of bits for subnet. His first example moves a /24 to a /26 address (2 bits) and only creates 2 subnets out of it. The reasoning is he states you cannot have a subnet of all 0's or all 1's, because 0 is the network ID and 1 is broadcast.

However, I don't find this to be true. My understanding is you cannot have a subnet + host ID to be all 0's or all 1's. There is a difference.

Using his logic, he eliminates an entire subnet to avoid having all 0's or all 1's, when instead it would make much more sense to eliminate a single host ID. In this example, he eliminates a subnet consisting approximately 64 machines to avoid a single situation. It would make much more sense to eliminate that one host ID that causes the issue.

Is my understanding correct? If so, how does the Net+ treat this situation? If I need 4 subnets, can it be created by using 2 bits or does it require 3?
Degrees: B.S. Computer Science, B.S. Mathematics

Certifications: Network+, Security+

In-Progress: M.S. Computer Science, CEH


  • earweedearweed Member Posts: 5,192 ■■■■■■■■■□
    For a better understanding of subnets see this thread
    No longer work in IT. Play around with stuff sometimes still and fix stuff for friends and relatives.
  • MentholMooseMentholMoose Member Posts: 1,525 ■■■■■■■■□□
    Your concern is detailed here:
    Subnet Zero and the All-Ones Subnet [IP Addressing Services] - Cisco Systems
    Using subnet zero for addressing was discouraged because of the confusion inherent in having a network and a subnet with indistinguishable addresses.
    Use of the all-ones subnet for addressing has been discouraged in the past because of the confusion inherent in having a network and a subnet with identical broadcast addresses.
    On Cisco exams they will explicitly say if you can use these two subnets. I don't know how it's handled on Network+.
    MCSA 2003, LFCS, LFCE (expired), VCP6-DCV
  • NetEngineer86NetEngineer86 Member Posts: 46 ■■□□□□□□□□
    That is true they are very VERY specific in the way they ask it.
    Kyle Hook
    AAS Computer Network Engineer

  • superman859superman859 Member Posts: 55 ■■□□□□□□□□
    Yes, I had read shortly before posting that the CCNA would expect the number of subnets to be 2^n, not 2^n - 2, thus allowing the all-zero and all-ones subnet. However, I haven't seen posts related to Net+ and how they treat it, but the textbook (for 2009 exam) talks about how they cannot be used - not sure if this is because of the Net+ exam or simply the authors call.

    That is what made me wonder.

    Does anyone know more specifics about the Net+ exam on this situation? I understand subnetting and the reasoning behind it as described in that Cisco article, but I'd hate to put the wrong answer on the Net+ exam if they expect something different.
    Degrees: B.S. Computer Science, B.S. Mathematics

    Certifications: Network+, Security+

    In-Progress: M.S. Computer Science, CEH
  • MosGuyMosGuy Member Posts: 195
    Unless the question clearly stated using all ones/zeros. I would default to using the 2^n - 2 formula personally. The possible answers should give a hint too, whether they round off to +/- 2.
    XPS 15: i7-6700HQ, 256 pcie ssd, 32 GB RAM, 2 GB Nvidia GTX 960m, windows 10 Pro

    Cert in progress: CCNA (2016 revision)
  • varelgvarelg Banned Posts: 790
    Another great resource on subnetting is the subnetting section in "Network Warrior". I had that book in my hands the other day and I really liked the explanation. It basically comes down to throwing away all formulas and remembering three things:
    -there are only 9 possible values in any subnet mask octet.
    -you can move host bits only to the left, anything else is invalid
    -all results are divisible by 256.
    A very important concept to have in mind is the ratio. At least that is what the author calls when you calculate number of hosts per subnet, for example ratio 8:32 means "8 subnets with 32 hosts each".
    Beside getting that book, you could consult only that portion of "Network Warrior" on Google Books...
  • rage_hograge_hog Banned Posts: 42 ■■□□□□□□□□
    There are several labs in the Cisco Academy that require the use of subnet0. However, at no point is it said that this is what is needed. You must figure it out on your own. By the way the 2^n-2 is the standard. If I ran across someone using subnet0 in a production environment without a DAMN good reason I would be talking some smack about it.

    Similar to cable standards. I once cabled a campus and nothing worked??? I figured out that some ******* was going around behind me and changing all my ends to his own "special" color code. In the end I cabled a campus twice. Now do that with the network addressing and I might even smack someone in the lip.

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