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How to manage serial numbers

Discussion in 'Embedded' started by pozzugno, Apr 10, 2014.

  1. pozzugno

    David Brown Guest

    It is certainly possible for it to happen, but only if you are using two
    cards from the same supplier with the same fixed MAC address. I have
    certainly come across - and even made - cards where the MAC address was
    compiled into the software or chosen from a couple of options using DIP
    switches. These cards would not work together on the same network. For
    the cards I made, that was perfectly acceptable behaviour - and having
    the same MAC address meant getting the same DHCP address during testing
    and development, which was an advantage.

    If you are talking about a conflict between two independent cards, then
    the chances of that happening are /really/ small. It is not impossible,
    of course.
    Yes. And if the customer expects to be able to use the cards in a
    random network, then you have to provide a unique address or at least an
    address that is highly unlikely to be a collision. (You can never be
    /sure/ your address is unique - someone else might have randomly picked
    /your/ address - and even if it is not your fault, it might end up being
    your problem.)
    I don't disagree with you here - that is undoubtedly best practice. It
    is just not the only practice used, and remember that "good enough" is
    often good enough - you don't always have to go for "best".
    If someone replaces the switch in the production line without checking
    the relevant documentation and consulting the relevant people involved,
    then we have a bigger problem than just a MAC address conflict. That's
    what procedures and quality control rules are for. (It's also why there
    are clear labels on the switch in question, in case someone makes a
    Yes, I am ignoring IPv6 in this case - because I am not using IPv6 here.

    And even if I were using IPv6, it would be perfectly possible to use it
    while the cards all had the same MAC address but different VLANs. IPv6
    is not /that/ different from IPv4, and the only MAC address connection
    is that MAC addresses are sometimes used to give an IPv6 address to a
    port - but there is nothing hindering the DHCP server from giving each
    card a different IPv6 address on the different VLANs, just as it
    currently does with IPv4 addresses.
    David Brown, Apr 14, 2014
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  2. pozzugno

    Les Cargill Guest

    IN this case, the MAC address was the default from whatever software
    brought the thing up. You had to use the equivalent of
    ifconfig to assign a MAC address.

    I was all totally "whut?"? :) But it was exactly as you say.
    Yes, they will not :)
    This is also, unfortunately, true.

    At least in my case, I'd expect the person changing a switch to
    be more in over their head than someone assigning MAC addresses.

    For this and other reasons, we don't spec managed switches at all -
    a WalMart special has to work out of the box. Our guys are great
    (they are very, very sharp ) , but we have to do some
    soldier-proofing for 'em. Any risk we can eliminate... this may
    happen once a decade.
    I agree - I haven't done that either. From what I *have* seen
    the nodes all have the IPv6 address that happens to be equivalent to the
    MAC address.

    Oh, the irony of DHCP for IPv6 :)
    Les Cargill, Apr 15, 2014
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  3. pozzugno

    David Brown Guest

    I have seen some systems like that. The information is usually hidden
    deep within the manual, in places no one looks until they are absolutely
    In this case, the cards were controlling drilling machines some 12m
    long, weighing several tons, and backed up by large hydraulic systems.
    You'd notice if someone else connected another one to your network!
    We don't often use managed switches either (except on the main network
    for VLANs). But in this particular case, it was a huge advantage - we
    don't assign real MACs until the second stage of programming, and a
    managed switch with VLANs lets us do lots of cards in parallel. So it
    was a definite active decision to use this setup. Normally I'd agree
    with you - when you can use simple generic parts, use simple generic
    parts, as it makes life much easier when something goes wrong. Here the
    solution is just to have a pre-programmed backup switch on hand.
    DHCP is used for many things, not just giving an IPv4 address, so it
    will still be useful with IPv6. And there are many reasons to want to
    control IP addresses, as well as connect DNS servers and DHCP servers
    together (or use dnsmasq, as I usually do) to control naming and IP. So
    no IPv6 system is going to stop /me/ from using DHCP!
    David Brown, Apr 15, 2014
  4. Am 10.04.2014 17:29, schrieb :
    None that i know of. I'm wondering too, because a lot of people do this.
    Think of requesting a block of serial numbers. First production line
    gets 1 to 1000, second line 1001 to 2000 and so on. If you can afford
    dropping single numbers, you can skip and use the next number after each
    programming attempt. You have to think about bad units. How do you
    retrieve that number, especially when it comes back for service. A
    printed serial number comes in handy.
    Is the testing done while on the programming fixture?
    If not, the device should send its serial number to the database, as
    this is the time you can increment stock count.

    Maybe you should program all serial numbers as 'FF' and the device
    should treat this as invalid and request a new number.
    Gunther Mannigel, Apr 15, 2014
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