VLSM question please?

Discussion in 'Cisco' started by daytime, Jan 1, 2007.

  1. daytime

    daytime Guest

    Hello-am unsure whether this is valid
    If I use a class C address and use the first subnet with a /26 mask
    > > 2nd subnet uses /27
    > > 3rd subnet uses /28
    > > 4th subnet uses /29
    > >
    > > which if I am correct takes me to .120
    > >
    > > can I then use a /26 address again? taking me to .184 ?
    > >Or do I have to use /30 from then on?
    > > TIA
    daytime, Jan 1, 2007
    #1
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  2. In article <>,
    "daytime" <> wrote:

    > Hello-am unsure whether this is valid
    > If I use a class C address and use the first subnet with a /26 mask
    > > > 2nd subnet uses /27
    > > > 3rd subnet uses /28
    > > > 4th subnet uses /29
    > > >
    > > > which if I am correct takes me to .120
    > > >
    > > > can I then use a /26 address again? taking me to .184 ?
    > > >Or do I have to use /30 from then on?
    > > > TIA


    You can use a /26, but it has to start at a multiple of 64, so the
    available ones are .128/26 and .192/26.

    --
    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, Jan 3, 2007
    #2
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  3. daytime

    daytime Guest

    I understand-is it possible to start off with a /30 address and then
    work backwards ?
    example -- /29 /28 /27 /26

    0-3
    4-11
    12-27
    28-59
    60-124 ?

    I am trying to cut down on wasting addresses-or is this just the nature
    of the beast?

    btw many thanks for answering.
    Barry Margolin wrote:
    > In article <>,
    > "daytime" <> wrote:
    >
    > > Hello-am unsure whether this is valid
    > > If I use a class C address and use the first subnet with a /26 mask
    > > > > 2nd subnet uses /27
    > > > > 3rd subnet uses /28
    > > > > 4th subnet uses /29
    > > > >
    > > > > which if I am correct takes me to .120
    > > > >
    > > > > can I then use a /26 address again? taking me to .184 ?
    > > > >Or do I have to use /30 from then on?
    > > > > TIA

    >
    > You can use a /26, but it has to start at a multiple of 64, so the
    > available ones are .128/26 and .192/26.
    >
    > --
    > 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 ***
    daytime, Jan 3, 2007
    #3
  4. daytime

    Guest

    In article <>, "daytime" <> writes:
    > I understand-is it possible to start off with a /30 address and then
    > work backwards ?
    > example -- /29 /28 /27 /26
    >
    > 0-3


    Yes, that's fine. But that's a /30.

    > 4-11


    That's not fine. That's a /29 (8 hosts) but it's aligned on a /30
    (multiple of 4) boundary. Try 8-15 instead.

    > 12-27


    That's not fine. That's a /28 aligned on a /30 boundary. Use 16-31

    > 28-59


    And that's a /27 aligned on a /30 boundary. Use 32-63

    > 60-124 ?


    That's a /26 aligned on a /30 boundary. Use 64-127

    If your subnet size is 2^n then it has to start on a 2^n boundary.

    You can carve the space up any way you please as long as you stick
    to that rule.

    There are sixteen ways to carve a /25 up into a /26, a /27, a /28,
    a /29 and two /30's.

    [Where do you want the /26 -- low half 0-63 or high half 64-127?
    Given that, where do you want the /27 -- low half or high half?
    Given that, where do you want the /28 -- low half or high half?
    Given that, where do you want the /29 -- low half or high half?
    Given that, the remaining space gets split into the two /30's]
    , Jan 3, 2007
    #4
  5. daytime

    daytime Guest

    wrote:
    > In article <>, "daytime" <> writes:
    > > I understand-is it possible to start off with a /30 address and then
    > > work backwards ?
    > > example -- /29 /28 /27 /26
    > >
    > > 0-3

    >
    > Yes, that's fine. But that's a /30.
    >
    > > 4-11

    >
    > That's not fine. That's a /29 (8 hosts) but it's aligned on a /30
    > (multiple of 4) boundary. Try 8-15 instead.
    >
    > > 12-27

    >
    > That's not fine. That's a /28 aligned on a /30 boundary. Use 16-31
    >
    > > 28-59

    >
    > And that's a /27 aligned on a /30 boundary. Use 32-63
    >
    > > 60-124 ?

    >
    > That's a /26 aligned on a /30 boundary. Use 64-127
    >
    > If your subnet size is 2^n then it has to start on a 2^n boundary.
    >
    > You can carve the space up any way you please as long as you stick
    > to that rule.
    >
    > There are sixteen ways to carve a /25 up into a /26, a /27, a /28,
    > a /29 and two /30's.
    >
    > [Where do you want the /26 -- low half 0-63 or high half 64-127?
    > Given that, where do you want the /27 -- low half or high half?
    > Given that, where do you want the /28 -- low half or high half?
    > Given that, where do you want the /29 -- low half or high half?
    > Given that, the remaining space gets split into the two /30's]


    Its just clicked now!!

    If I start with a /30 then I must use at least 2 subnets (to avoid
    wastage) before going on to the /29.

    Whereas if I start off big -/26 ,I can then "carve up "(nice
    description) the next subnet.

    Many many thanks for clearing that up--I dont work in IT yet ,but in
    your experience how are
    subnets usually worked out?
    Is it Big to Small or vice versa.?

    TIA
    daytime, Jan 3, 2007
    #5
  6. daytime

    Sam Wilson Guest

    In article <>,
    "daytime" <> wrote:

    > Hello-am unsure whether this is valid
    > If I use a class C address and use the first subnet with a /26 mask
    > > > 2nd subnet uses /27
    > > > 3rd subnet uses /28
    > > > 4th subnet uses /29
    > > >
    > > > which if I am correct takes me to .120
    > > >
    > > > can I then use a /26 address again? taking me to .184 ?
    > > >Or do I have to use /30 from then on?
    > > > TIA


    This may help (there are plenty of similar things around, just that this
    is the one I made myself).

    <http://www.ucs.ed.ac.uk/~ercm20/byte-table.html>

    Sam
    Sam Wilson, Jan 10, 2007
    #6
  7. daytime

    daytime Guest

    Sam Wilson wrote:
    > In article <>,
    > "daytime" <> wrote:
    >
    > > Hello-am unsure whether this is valid
    > > If I use a class C address and use the first subnet with a /26 mask
    > > > > 2nd subnet uses /27
    > > > > 3rd subnet uses /28
    > > > > 4th subnet uses /29
    > > > >
    > > > > which if I am correct takes me to .120
    > > > >
    > > > > can I then use a /26 address again? taking me to .184 ?
    > > > >Or do I have to use /30 from then on?
    > > > > TIA

    >
    > This may help (there are plenty of similar things around, just that this
    > is the one I made myself).
    >
    > <http://www.ucs.ed.ac.uk/~ercm20/byte-table.html>
    >
    > Sam


    Thanks alot for that-very easy to understand.
    Will be using it for along time to come !
    daytime, Jan 10, 2007
    #7
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