IP/Networking... 70-291.

Discussion in 'Server Networking' started by Robert Bollinger, Oct 24, 2007.

  1. Hello All -

    I am trying to understand that following information that comes from the
    70-291 book. this is on chapter 2-33 just above the figure 2.5.

    It says this: if i have 131.107.0.0/16 ... that i can subnet that like this:

    131.107.1.0 /24 << Router A, 131.107.2.0 << Router B, 131.107.25.0 /24

    This i understand just fine... however...

    It then says... Hosts external to the orginization then access hosts within
    the orginization using the /16 address.

    How can that be? how is it possibile for an external host (i.e. not on my
    sunbet) to access my internal hosts (using fully routeable ip's) with the
    wrong subnet mask?

    I am using /24 internall and they would be connecting to a /16 externally?

    Pelase advise.. and also i know that i am not the only person with this
    book, i have the ms press 70-291 second edition book.

    Thanks for all your help.

    Robert
     
    Robert Bollinger, Oct 24, 2007
    #1
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  2. It is called SuperNetting.

    [Router #1]
    |
    |
    |
    |
    [Router #2]--------<LAN 131.107.1.0/24>
    |
    |
    <LAN 131.107.2.0/24
    |
    |
    [Router #3]----------<LAN 131.107.25.0/24

    Since all three LANs "fit" inside of 131.107.0.0/16 the [Router #1] only has
    to know to send any thing contained within 131.107.0.0/16 to the [Router
    #2]

    The [Router #2] will already know what to do with it from that point.

    So you are "SuperNetting" all three LANs into a single route between [Router
    #1] and [Router #2].

    That is how the Internet works. Everything is SuperNetted. It doesn't
    really "break down" until it gets between the ISPs and thier customers.
    There are "billions" of routes on the Internet,...there is no way every
    router on the Internet can maintain all that in a routing table,...so
    everything is grouped together into a fewer number of routes containing
    *huge* blocks of addresses. So the router only cares about which "next hop"
    router to send traffic where the destination is a member of one particular
    "huge block" of addresses. Once sent it doesn't worry about it after
    that,...the next decision falls on the router that received it,...and the
    process repeats over and over and over.

    Think about it,...an IP Segment is not supposed to get over 250-300 hosts
    because performance starts to suffer from the excessive broadcasts. That is
    covered by the /24 bit mask that gives 254 Hosts. So what are all the other
    lower bit masks for? They are for SuperNetting upstream of the final
    destination to keep routing tables small and efficient. But when it nears
    the destination the IP Segments get broken down smaller and smaller until
    they get down to 254 hosts or less (masks of /24 bits or higher)

    --
    Phillip Windell
    www.wandtv.com

    The views expressed, are my own and not those of my employer, or Microsoft,
    or anyone else associated with me, including my cats.
    -----------------------------------------------------
     
    Phillip Windell, Oct 24, 2007
    #2
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  3. In
    I think Phillip pretty much explained it. The /24 nets are part of the whole
    /16, which you essentially are supernetting the /24's together as one /16.

    I would like to also point out that if you come in on a /16 of 131.107.0.0,
    such as through a VPN, you will be able to communicate to all three of these
    subnets. You can test it yourself with two workstations, with 131.107.1.2/24
    and 131.107.1.3/16. They will communicate.

    --
    Regards,
    Ace

    This posting is provided "AS-IS" with no warranties or guarantees and
    confers no rights.

    Ace Fekay, MCSE 2003 & 2000, MCSA 2003 & 2000, MCSE+I, MCT,
    MVP Microsoft MVP - Directory Services
    Microsoft Certified Trainer

    Infinite Diversities in Infinite Combinations

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    Ace Fekay [MVP], Oct 25, 2007
    #3
  4. SO here goes...

    I assign... the following to my first router that i got from my t3 provier.
    (Cisco 4700)

    External Fully routable - 131.107.0.1
    Subnet mask: 255.255.0.0
    Gateway: My isps gateway for this network. say .254

    Then i decide to assign this in my routers lan side for ethernet

    131.107.5.1
    255.255.255.0
    What gateway goes here?

    Then i decide to assign this in my routers lan side for eth/0
    131.107.10.1
    255.255.255.0
    what gateway goes here?

    I now understand what iw as doing wrong before. i was tryin to skip the
    "router step" and was assigning 131.107.1.0/16 (system a) then
    131.107.10.0/24 (system b).

    they would not communicatie, so clearly i had that wrong.

    In your scenario below, does router #2 have all the routes for that subent
    and therefore does all of the "routing internally"?

    if on my network i have 2 subnets 131.107.0.0/16 (default subnet) (which i
    own) and then i subnet that to 131.107.1.0/24, when any machine on the
    131.107.1.0/24 network communicates outside of its own network why the "hosts
    external to the /16" respond to the /16?

    Pleaes advise...

    Thank you,

    Robert


     
    Robert Bollinger, Oct 25, 2007
    #4
  5. There is no way I am going to be able to follow all that with numbers being
    thrown around all over. Here is my diagram again with the Gateways
    explained.

    [Router #1]
    |
    |
    |
    |
    [Router #2]--------<LAN 131.107.1.0/24>
    |
    |
    <LAN 131.107.2.0/24
    |
    |
    [Router #3]----------<LAN 131.107.25.0/24


    Router #1
    1. No way for me to know the Gateway of it. The Gateway of it will be
    whatever Router is farther "outbound" from it which isn't part of the
    Diagram.
    2. It has a Static Route telling it to use Router #2 for the
    131.107.0.0/16 segment

    Router #2
    1. This one uses Router #1 as the Gateway
    2. This one has a Static Route telling it to use Router #3 for the
    131.107.25.0/24 segment.

    Router #3
    1. This one uses Router #2 as the Gateway
    2. <nothing>


    --
    Phillip Windell
    www.wandtv.com

    The views expressed, are my own and not those of my employer, or Microsoft,
    or anyone else associated with me, including my cats.
    -----------------------------------------------------

     
    Phillip Windell, Oct 25, 2007
    #5
  6. Ok... I think that i understand now.

    all i am doing is taking /16, supernetting it then taking those supernetted
    addresses assigning them to different routers (and then dhcp, to assign to
    the clients) then allowing those clients to access the internet (through the
    primary router attached to the t3).

    it doesnt really matter what is what as long as a client has an ip (within
    the supernetted range) and as long as that client has a gateway. (for the
    next hop and so on).

    Thanks for your help...

    Any more input on that?

    Robert

     
    Robert Bollinger, Oct 25, 2007
    #6
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