what's the ' ip zero-subnet ' command for ?

Discussion in 'Cisco' started by JJ, Oct 24, 2004.

  1. JJ

    JJ Guest

    Hi all

    what's the ' ip zero-subnet ' command for ?

    Thanks.
     
    JJ, Oct 24, 2004
    #1
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  2. :what's the ' ip zero-subnet ' command for ?

    In the days before CIDR, if you had a set of subnets induced upon a
    base network address, then the first and last of those subnets
    were reserved. ip subnet-zero tells the router that those subnets
    are not reserved.

    For example, if you were using subnets of a class C address
    (say, 192.168.12.0) induced by the subnet mask 255.255.255.192
    (that's 64 addresses at a chunk), then you would have four
    subnets generated, 192.168.12.0-63, 192.168.12.64-127,
    192.168.12.128-191, and 192.168.12.192-255 . However, before CIDR
    you would not be allowed to use the first (192.168.12.0-63)
    or last (192.168.12.192-255), leaving you with only two
    usable subnets (192.168.12.64-127 and 192.168.12.128-191).


    I've heard that this is something that the Cisco certification
    exams love to trip people up on -- ask how many subnets there are
    of a certain address of a certain size, and anyone who doesn't
    reduce the "obvious" count by 2 because of the unusable ones is
    considered to be wrong. "ip subnet-zero" is not the default,
    and the cert exams like to push the edge of what the behaviour
    would be under any defaults that they do not specifically state
    have been changed.
     
    Walter Roberson, Oct 24, 2004
    #2
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  3. JJ

    Brian V Guest

    for the 0 and 255 subnet...all zero's or all 1's

    ie:
    10.0.0.0 255.255.255.0
    or
    10.0.255.0 255.255.255.0
     
    Brian V, Oct 24, 2004
    #3
  4. JJ

    JJ Guest

    Thanks.
    It's a very detail explanation.
     
    JJ, Oct 24, 2004
    #4
  5. JJ

    AnyBody43 Guest

    It is now.

    http://www.cisco.com/univercd/cc/td.../ios123/123cgcr/ipras_r/ip1_i2g.htm#wp1081869
    says:
    "ip subnet-zero
    To enable the use of subnet 0 for interface addresses and routing
    updates, use the ip subnet-zero command in global configuration mode.
    To restore the default, use the no form of this command.

    ip subnet-zero

    no ip subnet-zero

    Syntax Description
    This command has no arguments or keywords.

    Defaults
    Enabled"

    It has I think been the default for a while.
     
    AnyBody43, Oct 25, 2004
    #5
  6. :> > considered to be wrong. "ip subnet-zero" is not the default,

    :It is now.

    :http://www.cisco.com/univercd/cc/td.../ios123/123cgcr/ipras_r/ip1_i2g.htm#wp1081869
    :says:
    :"ip subnet-zero
    :To enable the use of subnet 0 for interface addresses and routing
    :updates, use the ip subnet-zero command in global configuration mode.
    :To restore the default, use the no form of this command.

    But if is on by default, and "no ip subnet-zero" restores the default,
    then the implication is you cannot turn it off. I don't think
    that is supposed to be the case, so the documentation you pointed
    to is likely wrong.
     
    Walter Roberson, Oct 25, 2004
    #6
  7. On 25 Oct 2004 15:51:15 GMT, -cnrc.gc.ca (Walter Roberson) wrote:

    ~ In article <>,
    ~
    ~ :> > considered to be wrong. "ip subnet-zero" is not the default,
    ~
    ~ :It is now.
    ~
    ~ :http://www.cisco.com/univercd/cc/td.../ios123/123cgcr/ipras_r/ip1_i2g.htm#wp1081869
    ~ :says:
    ~ :"ip subnet-zero
    ~ :To enable the use of subnet 0 for interface addresses and routing
    ~ :updates, use the ip subnet-zero command in global configuration mode.
    ~ :To restore the default, use the no form of this command.
    ~
    ~ But if is on by default, and "no ip subnet-zero" restores the default,
    ~ then the implication is you cannot turn it off. I don't think
    ~ that is supposed to be the case, so the documentation you pointed
    ~ to is likely wrong.

    It looks like that documentation is indeed wrong, I'm afraid.

    It states there that, by default, "ip subnet-zero" is enabled.
    However, I checked, and as of 12.3(11)T, "ip subnet-zero" is
    still the default.

    The fact that both "ip subnet-zero" and "no ip subnet-zero" are
    visible in the configuration does imply that we may be plotting
    to change the default in a future release.

    What IS correct is that "no ip subnet-zero" restores the default
    - although what would be MORE correct would be to state that
    the command "default ip subnet-zero" restores the default ... as
    it is the case that, in general, the "default" command can be counted
    on to restore any command to its default value, regardless of whether
    it is enabled or disabled by default.

    I'll put this on my list of stuff to get fixed.

    Aaron
     
    Aaron Leonard, Oct 25, 2004
    #7
  8. JJ

    AnyBody43 Guest

    I didn't read the whole entry, I was satisfied with:-

    "Defaults Enabled"

    which matched with my existing understanding
    and my own view of the meaning of 'default' which is
    something like "unconfigured default" and did not consider
    'IOS config file default' which is the condition where no
    config entry is displayed. I had never even heard of the IOS
    config command "default ... ...".


    Anyway:-


    (C837-K9O3SY6-M), Version 12.3(4)T3, RELEASE SOFTWARE (fc3)

    wr erase
    reload

    .. . . . .

    Router#sh run | inc subnet
    ip subnet-zero


    Router#conf t
    Router(config)#no ip subnet-zero
    Router#sh run | inc subnet
    no ip subnet-zero


    Router#conf t
    Router(config)#ip subnet-zero
    Router#sh run | inc subnet
    ip subnet-zero


    Router#conf t
    Router(config)#default ip subnet-zero
    Router#
    sh run | inc subnet
    no ip subnet-zero
    Router#


    Router#conf t
    Router(config)#default ip subnet-zero
    Router#sh run | inc subnet
    no ip subnet-zero
    Router#


    It seems that the "default command" does not return the
    "ip subnet-zero" state to the "router unconfigured" state
    which is what I would expect.

    (fc4) coming soon?<g>
     
    AnyBody43, Oct 26, 2004
    #8
  9. ~ > ~ :> > considered to be wrong. "ip subnet-zero" is not the default,
    ~ > ~
    ~ > ~ :It is now.
    ~ > I'll put this on my list of stuff to get fixed.
    ~ >
    ~ > Aaron

    ( where I assert that "no ip subnet-zero" is still the default
    as of 12.3(11)T, [although I typoed it and said that "ip subnet-zero"
    is the default], so our documentation that "ip subnet-zero"
    is the default is wrong ... )

    ~ I didn't read the whole entry, I was satisfied with:-
    ~
    ~ "Defaults Enabled"
    ~
    ~ which matched with my existing understanding
    ~ and my own view of the meaning of 'default' which is
    ~ something like "unconfigured default" and did not consider
    ~ 'IOS config file default' which is the condition where no
    ~ config entry is displayed. I had never even heard of the IOS
    ~ config command "default ... ...".
    ~
    ~
    ~ Anyway:-
    ~
    ~
    ~ (C837-K9O3SY6-M), Version 12.3(4)T3, RELEASE SOFTWARE (fc3)
    ~
    ~ wr erase
    ~ reload
    ~
    ~ . . . . .
    ~
    ~ Router#sh run | inc subnet
    ~ ip subnet-zero
    ~
    ~
    ~ Router#conf t
    ~ Router(config)#no ip subnet-zero
    ~ Router#sh run | inc subnet
    ~ no ip subnet-zero
    ~
    ~
    ~ Router#conf t
    ~ Router(config)#ip subnet-zero
    ~ Router#sh run | inc subnet
    ~ ip subnet-zero
    ~
    ~
    ~ Router#conf t
    ~ Router(config)#default ip subnet-zero
    ~ Router#
    ~ sh run | inc subnet
    ~ no ip subnet-zero
    ~ Router#
    ~
    ~
    ~ Router#conf t
    ~ Router(config)#default ip subnet-zero
    ~ Router#sh run | inc subnet
    ~ no ip subnet-zero
    ~ Router#
    ~
    ~
    ~ It seems that the "default command" does not return the
    ~ "ip subnet-zero" state to the "router unconfigured" state
    ~ which is what I would expect.

    Now I'm REALLY confused :)

    ~ (fc4) coming soon?<g>

    It had better not be! The fc<n> value is only bumped up while
    the IOS version is in internal testing ... e.g. 12.3(4)T3 (fc3)
    means that while we were testing 12.3(4)T3 (fc1) we found a showstopper
    bug and so built (fc2) then found another showstopper bug and then
    built (fc3) which is what you are running now (unless you got some
    code that escaped from our lab.) Once an IOS release is posted to
    CCO, the fc<n> value is never incremented; a rebuild from this
    branch would appear as 12.3(4)T4 (fc<n>).

    Cheers,

    Aaron
     
    Aaron Leonard, Oct 26, 2004
    #9
  10. JJ

    jantu

    Joined:
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    assign last subnet addresses while using no ip subnet-zero command

    hi,

    I use these command in IOS 1.x and 2.x. but when i use no ip subnet-zero command i found i m not able to use first subnet addresses but i can able to assign last subnet addresses.
    How It is possible....




     
    jantu, Jan 28, 2008
    #10
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