One machine seeing broadcast traffic from a different logical IP network

Discussion in 'Windows Networking' started by Spin, Sep 29, 2008.

  1. Spin

    Spin Guest

    Gurus,

    I thought this was odd. I have a small network of machines all running on a
    192.168.1.x/24 subnet. On the same physical segment, I have another machine
    who's TCP/IP settings I switched to 10.0.0.2/8. Afterwards, this machine
    could no longer initiate point-to-point communications with any machine on
    the 192.168.1.x subnet as expected. I also installed Wireshark on this
    machine to see what kind of traffic this machine would pickup. Since it is
    on a completely different logical IP network, I expected to see NO packets.
    Much to my surprise, I was seeing broadcast traffic. Stuff like Browser
    traffic along the lines of host announcements and Domain/workgroup
    announcements. I was hypothesizing that I wouldn't see any traffic coming
    across it's NIC. I'm a bit confused.
     
    Spin, Sep 29, 2008
    #1
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  2. Broadcast traffic is sent to all nodes on the physical segment, it's not
    specific to an IP subnet.
     
    Barry Margolin, Sep 29, 2008
    #2
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  3. Spin

    Spin Guest

    I am now duly informed. Thanks!!!!
     
    Spin, Sep 29, 2008
    #3
  4. Any ethernet broadcast traffic will be received by the NIC if it's on
    its wire. It will then be up to the OS to decide whether to pass that
    broadcast traffic to higher levels. Some network cards have broadcast
    filters, in which case only traffic that passes the filter will be
    handed to the OS.

    If the traffic was a broadcast UDP datagram send to 255.255.255.255,
    it should be fully processed by the OS. If it was sent to another
    subnet's broadcast address, say, 192.168.1.255, it should be rejected
    by the OS.

    Wireshark will likely see any packets the network card receives,
    whether or not the OS would process them. If it places the NIC into
    promiscuous mode, it will see anything on the wire.

    You can use VLANs if you need a higher level of isolation between
    logical subnets on the same physical network.

    DS
     
    David Schwartz, Sep 29, 2008
    #4
  5. Spin

    Spin Guest

    Can you use VLANs in VMWare Workstation 6.0?
     
    Spin, Sep 30, 2008
    #5
  6. There are several different ways you could try to do it. One would be
    to configure the VLANs in the virtual machine. But that would rely on
    VMWare passing the VLAN tag to the bridged interface. Another way
    would be to do it in the host, for example, you could bridge the
    VMWare guest to a dummy interface that's bridged to a VLAN on the main
    interface.

    I've never tried it, nor have I ever seen any documentation that
    discussed whether that would work. I'm sure you could get it to work
    with enough hackery, but there's probably a simple and supported way
    to do it.

    DS
     
    David Schwartz, Sep 30, 2008
    #6
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