Pure IP & ARP broadcasts

Discussion in 'Windows Networking' started by =?Utf-8?B?c3c=?=, May 3, 2004.

  1. Hi, I'm only using IP on my network and I have several W2K DNS servers yet I still see quite a lot of ARP broadcasts(approx 40%). If DNS maintains a list of all IP and Mac addresses why the need for ARP broadcasts? Am I missing something?

    Many thanks
     
    =?Utf-8?B?c3c=?=, May 3, 2004
    #1
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  2. DNS maintains an IP to hostname mapping. no MAC address info is retained.
    ARP is used to resolve an IP to a MAC.
    --
    --
    Dusty Harper
    Microsoft Corporation
    ----------------------------------------------------------------------------
    This posting is provided "AS IS", with NO warranties and confers NO rights
    ----------------------------------------------------------------------------

    I still see quite a lot of ARP broadcasts(approx 40%). If DNS maintains a
    list of all IP and Mac addresses why the need for ARP broadcasts? Am I
    missing something?
     
    Dusty Harper {MS}, May 3, 2004
    #2
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  3. Hello again. You're keeping me busy :)

    That's an interesting question to me. I think a lot of people don't
    understand what is behind it.

    You see a LAN (Ethernet) does not really communicate by IP# like many may
    think. It actually communicates via the MAC address of the Nics (aka Layer2
    address, hardware address, ethernet address, all mean the same thing). The
    ARP request is what happens when the IP# is known but the MAC is not, since
    the MAC is required to communicate the MAC must be discovered,...that is
    what the ARP request does.

    IP#s really only have two purposes:
    #1. Provide the mechanism for Layer3 routing. Routers make routing
    descisions based to the "network" portion of the IP# by comparing it to
    their Routing Table.
    #2. Provide the means to find the MAC address. The IP# (including the "host"
    portion this time) is used in the ARP request to discover the MAC address.

    A host has a packet to send, it has the IP# and nothing else. It broadcasts
    an ARP request out onto the segment. If the owner of that IP# exists on the
    segment it responds with its MAC address. The sender then sends the packet
    to that MAC address. All done, everybody is happy.

    But if the owner of the IP# is not on that segment then the Router replies
    with its own IP#, in other words it "lies" to the sender (aka "Proxy Arp").
    The sender passes the packet to the Router's MAC address as if that was the
    destination. The router examine's its Routing Table using the IP# to find
    the proper port leading to the destination and broadcasts it own ARP
    request out that port and the whole process repeats again. If another Router
    is required then that second Router "lies" to the first Router and the whole
    process repeats again. This happens over and over until the broadcast hits
    the right segment and the "real" host responds to the ARP with its own MAC
    address and the packet is sent there an has finally made it "home".

    Also every host (Routers, Switches, PCs, Servers) on a network keeps a small
    "cache" of MAC/IP# pairs. If an entry exists in this cache it will send the
    packet to that MAC address without doing an "ARP" but if the entry has
    expired then it must do another "ARP".

    There may be a little more detail to the process than this, particularly
    when there are multpile routes with routers that are Default Gateway and
    those that aren't, I'm a little fuzzy on some of that myself. But it should
    give you an idea of what those ARPs are all about. They are normal, they are
    supposed to be there.

    --

    Phillip Windell [MCP, MVP, CCNA]
    www.wandtv.com

    I still see quite a lot of ARP broadcasts(approx 40%). If DNS maintains a
    list of all IP and Mac addresses why the need for ARP broadcasts? Am I
    missing something?
     
    Phillip Windell, May 3, 2004
    #3
  4. =?Utf-8?B?c3c=?=

    Bill Grant Guest

    One obvious reason why IP addresses cannot include the MAC address is
    that IP addresses are 32 bits, but MAC addresses are 48 bits.

     
    Bill Grant, May 4, 2004
    #4
  5. Many thanks Phillip for your comments, very helpful

    Thanks again.
     
    =?Utf-8?B?c3c=?=, May 4, 2004
    #5
  6. Is the next version of TCP/IP going to involve the MAC somehow? I've only
    briefly studied it and wasn't very interested so I forgot about everything I
    read.
     
    Phillip Windell, May 4, 2004
    #6
  7. =?Utf-8?B?c3c=?=

    Bill Grant Guest

    Phillip,

    The IPv6 address is 128 bits long, and the interface portion is 64
    bits - long enough to hold the current MAC addressses of 48 bits (and enough
    for the proposed "new" MAC addresses of 64 bits).
     
    Bill Grant, May 5, 2004
    #7
  8. Thank you sir..

    BTW - I was thinking last night that a few days ago you had posted a KB
    number for using RRAS as a LAN Router. Was the you? Do you have that number?
    I probably need to refresh my memory with it. I have been dealing with
    questions on RRAS in the form as you've probably seen and I'm becomming
    unsure of something. With a normal router there isn't a need for static
    roues to be entered when all the involved networks are directly connected to
    a port on the router. However with RRAS, is it different?..does it still
    need static routes entered for even directly connected networks? It
    sometimes doesnt' seem to behave as I'd expect some other brand of router
    for behave. My router experience had always been Cisco and HP.
     
    Phillip Windell, May 5, 2004
    #8
  9. Pardon my bad spelling and grammer this morning, I guess I haven't woke up
    yet.


    --

    Phillip Windell [MCP, MVP, CCNA]
    www.wandtv.com

     
    Phillip Windell, May 5, 2004
    #9
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