Unique IP address for PPP interface? - PPP

This is a discussion on Unique IP address for PPP interface? - PPP ; I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0 interfaces but the system I was given to work with -- which ostensibily works -- has the same address for both: eth0 Link encap:Ethernet HWaddr ...

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Thread: Unique IP address for PPP interface?

  1. Unique IP address for PPP interface?

    I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0
    interfaces but the system I was given to work with -- which ostensibily
    works -- has the same address for both:

    eth0 Link encap:Ethernet HWaddr 00:A0:1D:2E:2E:1F
    inet addr:10.93.0.190 Bcast:10.255.255.255 Mask:255.0.0.0
    UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
    RX packets:75 errors:0 dropped:0 overruns:0 frame:0
    TX packets:36 errors:0 dropped:0 overruns:0 carrier:0
    collisions:0 txqueuelen:100
    RX bytes:5360 (5.2 kb) TX bytes:2693 (2.6 kb)
    Base address:0xe00
    ....
    ppp0 Link encap:Point-Point Protocol
    inet addr:10.93.0.190 P-t-P:10.192.8.11 Mask:255.255.255.255
    UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
    RX packets:5 errors:0 dropped:0 overruns:0 frame:0
    TX packets:5 errors:0 dropped:0 overruns:0 carrier:0
    collisions:0 txqueuelen:3
    RX bytes:78 (78.0 b) TX bytes:72 (72.0 b)

    This seems quite wrong but if, as I'm told, this is working then it
    must be right and I'm confused. I don't know enough PPP to be sure.


  2. Re: Unique IP address for PPP interface?

    "Chris Nelson" writes:
    > I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0
    > interfaces but the system I was given to work with -- which ostensibily
    > works -- has the same address for both:


    The local address on a point-to-point link may be identical to one or
    more other local addresses. It's the remote address that must be
    unique.

    > eth0 Link encap:Ethernet HWaddr 00:A0:1D:2E:2E:1F
    > inet addr:10.93.0.190 Bcast:10.255.255.255 Mask:255.0.0.0

    [...]
    > ppp0 Link encap:Point-Point Protocol
    > inet addr:10.93.0.190 P-t-P:10.192.8.11 Mask:255.255.255.255


    That's fine. You have a distinct remote address. The system uses the
    remote address to distinguish among point-to-point interfaces, not the
    local address.

    > This seems quite wrong but if, as I'm told, this is working then it
    > must be right and I'm confused. I don't know enough PPP to be sure.


    It looks fine to me. What problem are you trying to solve?

    --
    James Carlson, KISS Network
    Sun Microsystems / 1 Network Drive 71.232W Vox +1 781 442 2084
    MS UBUR02-212 / Burlington MA 01803-2757 42.496N Fax +1 781 442 1677

  3. Re: Unique IP address for PPP interface?

    James Carlson wrote:
    > "Chris Nelson" writes:
    > > I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0
    > > interfaces but the system I was given to work with -- which ostensibily
    > > works -- has the same address for both:

    >
    > The local address on a point-to-point link may be identical to one or
    > more other local addresses. It's the remote address that must be
    > unique.


    Thanks.

    > ...
    > It looks fine to me. What problem are you trying to solve?


    I've got my PPP client and server sitting on a tiny little LAN on my
    desk and I'm trying to test PPP with a null modem cable. Maybe that's
    a bad thing. I can ping from the server to the client via PPP because
    the IP address chooses the ppp0 interface. But I can't ping from the
    client to the server because the server can be reached -- with the same
    IP -- on both eth0 and ppp0. I guess that situation is unlikely to
    arise in practice.


  4. Re: Unique IP address for PPP interface?

    "Chris Nelson" writes:
    > I've got my PPP client and server sitting on a tiny little LAN on my
    > desk and I'm trying to test PPP with a null modem cable. Maybe that's
    > a bad thing.


    That should work fine.

    > I can ping from the server to the client via PPP because
    > the IP address chooses the ppp0 interface. But I can't ping from the
    > client to the server because the server can be reached -- with the same
    > IP -- on both eth0 and ppp0. I guess that situation is unlikely to
    > arise in practice.


    Then just shut down eth0 for purposes of testing, or use different
    addresses.

    The situation you're talking about is really just fine from a
    networking point of view. Each system is doing *exactly* what it's
    supposed to do -- finding the best way to send packets to the other.
    That the decision is different in the two cases is pretty much
    immaterial.

    IP is supposed to get the packets through, not make testing easy. ;-}

    --
    James Carlson, KISS Network
    Sun Microsystems / 1 Network Drive 71.232W Vox +1 781 442 2084
    MS UBUR02-212 / Burlington MA 01803-2757 42.496N Fax +1 781 442 1677

  5. Re: Unique IP address for PPP interface?

    "Chris Nelson" writes:

    >James Carlson wrote:
    >> "Chris Nelson" writes:
    >> > I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0
    >> > interfaces but the system I was given to work with -- which ostensibily
    >> > works -- has the same address for both:

    >>
    >> The local address on a point-to-point link may be identical to one or
    >> more other local addresses. It's the remote address that must be
    >> unique.


    >Thanks.


    >> ...
    >> It looks fine to me. What problem are you trying to solve?


    >I've got my PPP client and server sitting on a tiny little LAN on my
    >desk and I'm trying to test PPP with a null modem cable. Maybe that's
    >a bad thing. I can ping from the server to the client via PPP because
    >the IP address chooses the ppp0 interface. But I can't ping from the
    >client to the server because the server can be reached -- with the same
    >IP -- on both eth0 and ppp0. I guess that situation is unlikely to
    >arise in practice.


    Yes, Just tell your server to route that address over ppp.
    The routing tables will always take the most specific address.

    route add -host 111.222.333.444 ppp0



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