How is this network possible?

Discussion in 'Cisco' started by zulalogl@hotmail.com, Apr 8, 2005.

  1. Guest

    Let A, B, C routers... and the interface IPs start with "198.30.30."


    --[A]"78"-----"79""77"------"76"[C]---


    I think 76,77,78,79 should all belong to the same subnet. How can they
    assign IPs like this?

    One thing that came to my mind is that, this could be an ethernet conn.
    and all interfaces face the same switched network. Is that a correct
    approach? Is there a way to these 2 links in different networks?? There
    is no such thing as "/31" prefix... right?

    Thanks in advance...
    , Apr 8, 2005
    #1
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  2. In article <>,
    wrote:

    > Let A, B, C routers... and the interface IPs start with "198.30.30."
    >
    >
    > --[A]"78"-----"79""77"------"76"[C]---
    >
    >
    > I think 76,77,78,79 should all belong to the same subnet. How can they
    > assign IPs like this?
    >
    > One thing that came to my mind is that, this could be an ethernet conn.
    > and all interfaces face the same switched network. Is that a correct
    > approach? Is there a way to these 2 links in different networks?? There
    > is no such thing as "/31" prefix... right?


    Yes, I believe Cisco does support /31 prefixes for point-to-point links.
    There's no need for broadcast addresses on these subnets.

    I believe there was an RFC a few years ago that made this officially OK.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    Barry Margolin, Apr 8, 2005
    #2
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  3. DigitalVinyl Guest

    Barry Margolin <> wrote:

    >In article <>,
    > wrote:
    >
    >> Let A, B, C routers... and the interface IPs start with "198.30.30."
    >>
    >>
    >> --[A]"78"-----"79""77"------"76"[C]---
    >>
    >>
    >> I think 76,77,78,79 should all belong to the same subnet. How can they
    >> assign IPs like this?
    >>
    >> One thing that came to my mind is that, this could be an ethernet conn.
    >> and all interfaces face the same switched network. Is that a correct
    >> approach? Is there a way to these 2 links in different networks?? There
    >> is no such thing as "/31" prefix... right?

    >
    >Yes, I believe Cisco does support /31 prefixes for point-to-point links.
    >There's no need for broadcast addresses on these subnets.
    >
    >I believe there was an RFC a few years ago that made this officially OK.


    I don't know if that one is true. Cisco supports address-less
    p-to-p's. You assign no address and when you reference the serial
    interface for a route is automatically sends to other side with no
    need for IP routing/switching.

    Since Cisco support no-ip-address on a link I'm not sure why they
    would bother supporting a /31 scheme, unless the no-ip-address isn't
    ratified beyond cisco.

    Even without an RFC, I think with the right statics it could be made
    to work with no problems. Networking can be very broken and still
    work--not work well, but it will appear to pass SOME traffic.

    DiGiTAL_ViNYL (no email)
    DigitalVinyl, Apr 8, 2005
    #3
  4. Brad Guest

    Brad, Apr 8, 2005
    #4
  5. papi Guest

    papi, Apr 8, 2005
    #5
  6. In article <>,
    DigitalVinyl <> wrote:

    > Since Cisco support no-ip-address on a link I'm not sure why they
    > would bother supporting a /31 scheme, unless the no-ip-address isn't
    > ratified beyond cisco.


    There are benefits to having unique IPs for each interface, rather than
    using "ip unnumbered". For instance, in a traceroute you can translate
    the IP to an interface, and you can ping specific interfaces to tell if
    they're up.

    /31 addressing was created because many ISPs were using /30 for this,
    thus wasting half the addresses.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    Barry Margolin, Apr 9, 2005
    #6
  7. Guest

    Guys, thank you very much...
    , Apr 10, 2005
    #7
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