two ip subnets in one hub possible?

Discussion in 'Cisco' started by John, Jan 9, 2004.

  1. John

    Sam Wilson Guest

    No problems with most of what you wrote - just a couple of niggles
    still... :)

    OK, so those devices are going to be disappointed some (a lot?) of the
    time. :)
    Uhhh - I've never noticed this except for when a device was configured
    with the wrong subnet mask. Any more details?

    Sam Wilson, Jan 19, 2004
    1. Advertisements

  2. John

    Hansang Bae Guest

    Only a host which believes that proxy ARP would be supported by

    This happens more often than people think with bigger boxes (Tandems,
    MF's, Older Unix boxes). Basically, you set your gateway to be
    yourself, and you'll happily arp for everyone in the world. No
    knowledge about routers supporting proxy arp is required. They just
    blindly arp for everyone.



    "Somehow I imagined this experience would be more rewarding" Calvin
    *************** USE ROT13 TO SEE MY EMAIL ADDRESS ****************
    Due to the volume of email that I receive, I may not not be able to
    reply to emails sent to my account. Please post a followup instead.
    Hansang Bae, Jan 19, 2004
    1. Advertisements

  3. John

    Sam Wilson Guest

    OK, I'd forgotten that trick. As I noted such machines will of course
    flounder when they don't have proxy ARP available.

    Sam Wilson, Jan 20, 2004
  4. John

    AnyBody43 Guest

    Likewise and since I had never seen it in action ...

    I also tried no default gw but that did not work.

    Here it is:
    XP PC with default gw of itself.

    + Frame: Base frame properties
    ETHERNET: ETYPE = 0x0806 : Protocol = ARP: Address Resolution Protocol
    + ETHERNET: Destination address : FFFFFFFFFFFF
    + ETHERNET: Source address : 000D9D4915E4
    ETHERNET: Frame Length : 42 (0x002A)
    ETHERNET: Ethernet Type : 0x0806 (ARP: Address Resolution Protocol)
    ETHERNET: Ethernet Data: Number of data bytes remaining = 28 (0x001C)
    ARP_RARP: ARP: Request, Target IP:
    ARP_RARP: Hardware Type = Ethernet (10Mb)
    ARP_RARP: Protocol Type = 2048 (0x800)
    ARP_RARP: Hardware Address Length = 6 (0x6)
    ARP_RARP: Protocol Address Length = 4 (0x4)
    ARP_RARP: Opcode = Request
    ARP_RARP: Sender's Hardware Address = 000D9D4915E4
    ARP_RARP: Sender's Protocol Address =
    ARP_RARP: Target's Hardware Address = 000000000000
    ARP_RARP: Target's Protocol Address =

    In the context of this discussion key points are:

    No IP header - not IP
    ETHERNET: Ethernet Type : 0x0806 (ARP: Address Resolution Protocol)
    IP is type 0x0800
    It has not been mentioned (I hope) that routers will only
    issue proxy arp responses for addresses that they have a route

    To generate the above I put a router on my network and
    created a loopback interface with address
    AnyBody43, Jan 20, 2004
  5. John

    Bernie Guest

    Possibly. My only point in all of this wasn't to comment on what is
    normal, or what is the best practice, but rather to clarify that it is
    possible on a multinet to have devices communicate even when the
    router isn't present. It was a tangent, but I thought that it was an
    interesting note to present.
    Well, I think I know how to spot a misconfigured subnet mask. Anyway,
    Hansang clarified how it can be done.

    Bernie, Jan 21, 2004
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.