when should i turn off ip subnet-zero ?

Discussion in 'Cisco' started by jh3ang, Aug 2, 2006.

  1. jh3ang

    jh3ang Guest

    Hi ? I'm wondering when or in what particular reason should i tun off
    ip subnet-zero ?

    I saw this statement:
    "ip subnet-zero //if router only has its subnet, it should not be used,
    so disable ip subnet zero"

    please tell me.. what does it mean ?
    is there any occasion to turn it off ?
     
    jh3ang, Aug 2, 2006
    #1
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  2. In article <>,
    jh3ang <> wrote:
    >Hi ? I'm wondering when or in what particular reason should i tun off
    >ip subnet-zero ?


    If you aren't running Sun 3 systems, you probably don't need to
    leave subnet 0 reserved.

    There used to be a reservation of the first subnet within a network,
    for use in broadcasting, because at the time either the first or last
    address in a range could be used as the broadcast address. The
    use of the first address as the broadcast address went away a long
    time ago, but Cisco equipment still reserves it in case you are
    running old equipment.
     
    Walter Roberson, Aug 2, 2006
    #2
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  3. jh3ang

    Nandan Guest

    Hi There,

    When you subnet a major network, you will get multiple subnets. e.g. If
    you subnet 192.168.1.0/24 into /26 subnets, then you will get following
    four subnets:
    192.168.1.0/26
    192.168.1.64/26
    192.168.1.128/26
    192.168.1.192/26

    If you turn off ip subnet-zero, then you will NOT be able to use subnet
    zero (in this case 192.168.1.0/26). I have always used ip subnet-zero
    command, and it has never created a problem for me. So I would never
    recommend anyone to turn it off.

    Hope this helps.

    Best Regards
    Nandan
     
    Nandan, Aug 3, 2006
    #3
  4. In article <>,
    Nandan <> wrote:

    >If you turn off ip subnet-zero, then you will NOT be able to use subnet
    >zero (in this case 192.168.1.0/26).


    Correct [in the context I snipped]

    >I have always used ip subnet-zero
    >command, and it has never created a problem for me.


    Okay, fine, a valid observation of your personal experience.

    >So I would never
    >recommend anyone to turn it off.


    But that statement tends to indicate that you do not understand
    why ip subnet-zero was invented. If you knew -why- subnet zero was
    not normally usable, then you would not say that you would "never"
    recommend that anyone would turn it off, because some day you may
    encounter a network that relies upon subnet zero being reserved.
     
    Walter Roberson, Aug 3, 2006
    #4
  5. In article <EKpAg.307255$iF6.70537@pd7tw2no>,
    (Walter Roberson) wrote:

    > In article <>,
    > Nandan <> wrote:
    > >So I would never
    > >recommend anyone to turn it off.

    >
    > But that statement tends to indicate that you do not understand
    > why ip subnet-zero was invented. If you knew -why- subnet zero was
    > not normally usable, then you would not say that you would "never"
    > recommend that anyone would turn it off, because some day you may
    > encounter a network that relies upon subnet zero being reserved.


    It was invented because of issues that existed over a decade ago, and
    are now obsolete. Unless you go into a shop that is still running
    10-year-old systems, you can safely forget about it. Unfortunately,
    Cisco is really slow at changing their default configuration settings.

    In other words, knowledge of this is about as useful as knowing how to
    administer Windows 3.x systems.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***
     
    Barry Margolin, Aug 4, 2006
    #5
  6. In article <>,
    Barry Margolin <> wrote:

    [ip subnet-zero]

    >It was invented because of issues that existed over a decade ago, and
    >are now obsolete. Unless you go into a shop that is still running
    >10-year-old systems, you can safely forget about it.


    Yup. Now turn the logic around: If you were to happen to go to a shop
    running old systems (or which couldn't afford to change after
    having run those systems for a long time), then you would need to
    know the command, and in those circumstances you would recommend
    its use.

    The poster I was replying to was saying that he would *never*
    recommend use of the command. He might never encounter those legacy
    systems (outside of a cisco cert exam ;-) ) but if he -does-
    happen to encounter them, how is he going to deal with the
    situation if he has denied himself use of the appropriate tool?
     
    Walter Roberson, Aug 4, 2006
    #6
  7. In article <soAAg.316247$IK3.306214@pd7tw1no>,
    (Walter Roberson) wrote:

    > The poster I was replying to was saying that he would *never*
    > recommend use of the command.


    And of course on the Internet, every word must be taken absolutely
    literally.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***
     
    Barry Margolin, Aug 4, 2006
    #7
  8. In article <>,
    Barry Margolin <> wrote:
    >In article <soAAg.316247$IK3.306214@pd7tw1no>,
    > (Walter Roberson) wrote:


    >> The poster I was replying to was saying that he would *never*
    >> recommend use of the command.


    >And of course on the Internet, every word must be taken absolutely
    >literally.


    On a technical newsgroup, when giving technical advice to others,
    "I would never" conveys a very different message than "I have never".
     
    Walter Roberson, Aug 6, 2006
    #8
  9. In article <DzoBg.323042$iF6.118216@pd7tw2no>,
    (Walter Roberson) wrote:

    > In article <>,
    > Barry Margolin <> wrote:
    > >In article <soAAg.316247$IK3.306214@pd7tw1no>,
    > > (Walter Roberson) wrote:

    >
    > >> The poster I was replying to was saying that he would *never*
    > >> recommend use of the command.

    >
    > >And of course on the Internet, every word must be taken absolutely
    > >literally.

    >
    > On a technical newsgroup, when giving technical advice to others,
    > "I would never" conveys a very different message than "I have never".


    I was referring to interpreting "never" as a literal absolute. Like if
    someone were to say "I would never kill someone", although in fact there
    are most likely some extreme situations where they actually would. Or,
    if you want a technical example, one might say "Never just cut power to
    a computer, you should always perform a graceful shutdown"; the
    qualification that this only applies when a graceful shutdown is
    possible is obvious.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***
     
    Barry Margolin, Aug 7, 2006
    #9
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