Same Subnet, different subnet mask - TCP-IP

This is a discussion on Same Subnet, different subnet mask - TCP-IP ; Hi, Is there any case where it would be useful to give 2 hosts in the same network (and subnet) a different subnet mask? I don't think there is, but one of our customers seems to be doing it and ...

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Thread: Same Subnet, different subnet mask

  1. Same Subnet, different subnet mask

    Hi,

    Is there any case where it would be useful to give 2 hosts in the same
    network (and subnet) a different subnet mask? I don't think there is,
    but one of our customers seems to be doing it and complains about
    intermittent communication problems (which would seem logical to me),
    but before we tell them their completely messing things up that way (I
    don't know exactly how their network is configured), I want to be sure.

    An example:
    Host1: 172.16.16.1/20
    Host2: 172.16.16.2/24
    Host3: 172.16.32.1/24

    I think:
    * Hosts 1 and 2 might very well be able to communicate, but would there
    be a reason not to use /20 as well?
    * Hosts 1 and 3 will have problems: 1 can see 3, but 3 will try the
    gateway to answer (causing perhaps the intermittent problems?)
    * 2 and 3 can only communicate using a gateway(router).

    Any ideas would be greatly appreciated!

    The Unlord.

  2. Re: Same Subnet, different subnet mask

    Edit:
    Host3: 172.16.32.1/24
    should be
    Host3: 172.16.17.1/24
    of course...

    The Unlord schreef:
    > Hi,
    >
    > Is there any case where it would be useful to give 2 hosts in the same
    > network (and subnet) a different subnet mask? I don't think there is,
    > but one of our customers seems to be doing it and complains about
    > intermittent communication problems (which would seem logical to me),
    > but before we tell them their completely messing things up that way (I
    > don't know exactly how their network is configured), I want to be sure.
    >
    > An example:
    > Host1: 172.16.16.1/20
    > Host2: 172.16.16.2/24
    > Host3: 172.16.32.1/24
    >
    > I think:
    > * Hosts 1 and 2 might very well be able to communicate, but would there
    > be a reason not to use /20 as well?
    > * Hosts 1 and 3 will have problems: 1 can see 3, but 3 will try the
    > gateway to answer (causing perhaps the intermittent problems?)
    > * 2 and 3 can only communicate using a gateway(router).
    >
    > Any ideas would be greatly appreciated!
    >
    > The Unlord.


  3. Re: Same Subnet, different subnet mask

    On Thu, 31 May 2007 10:03:00 +0200, The Unlord wrote:

    > Edit:
    > Host3: 172.16.32.1/24
    > should be
    > Host3: 172.16.17.1/24
    > of course...
    >
    > The Unlord schreef:
    >> Hi,
    >>
    >> Is there any case where it would be useful to give 2 hosts in the same
    >> network (and subnet) a different subnet mask? I don't think there is,
    >> but one of our customers seems to be doing it and complains about
    >> intermittent communication problems (which would seem logical to me),
    >> but before we tell them their completely messing things up that way (I
    >> don't know exactly how their network is configured), I want to be sure.
    >>
    >> An example:
    >> Host1: 172.16.16.1/20
    >> Host2: 172.16.16.2/24
    >> Host3: 172.16.32.1/24
    >>
    >> I think:
    >> * Hosts 1 and 2 might very well be able to communicate, but would there
    >> be a reason not to use /20 as well?
    >> * Hosts 1 and 3 will have problems: 1 can see 3, but 3 will try the
    >> gateway to answer (causing perhaps the intermittent problems?) * 2 and
    >> 3 can only communicate using a gateway(router).
    >>


    It CAN work, as long as you know exactly what you are doing, and what
    limitations occur. We run one LAN where a /23 is seen by some hosts as
    two /24s. There are good reasons for that but these are all to do with
    limitations those hosts.

    The situation you painted above can also work. It's crazy, I would not
    advise it, but it can work.

    I would advice to fire up wireshark (formally ethereal) and look what
    goes on at the wire to really see where the problem is.

    That or convince the client that although it can work (with limitations),
    it is not advisable because it is brittle, bound to fail somehow
    somewhere.

    M4

  4. Re: Same Subnet, different subnet mask

    The Unlord writes:
    > Is there any case where it would be useful to give 2 hosts in the same
    > network (and subnet) a different subnet mask? I don't think there is,


    The only case I can think of is with bridging by way of proxy ARP.
    It's an extraordinarily lame thing to do, very hard to get right, and
    falls apart when subjected to broadcast traffic.

    > Host1: 172.16.16.1/20
    > Host2: 172.16.16.2/24


    Ick. Get out the clue stick.

    --
    James Carlson, Solaris Networking
    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: Same Subnet, different subnet mask

    "James Carlson" wrote in message
    news:xoavvee99lq1.fsf@sun.com...
    > The Unlord writes:
    >> Is there any case where it would be useful to give 2 hosts in the same
    >> network (and subnet) a different subnet mask? I don't think there is,

    >
    > The only case I can think of is with bridging by way of proxy ARP.
    > It's an extraordinarily lame thing to do, very hard to get right, and
    > falls apart when subjected to broadcast traffic.
    >
    >> Host1: 172.16.16.1/20
    >> Host2: 172.16.16.2/24

    >
    > Ick. Get out the clue stick.
    >
    > --
    > James Carlson, Solaris Networking
    > 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


    Agreed. I'd really love to hear the response when they are asked why they
    did it this way.

    Jim



  6. Re: Same Subnet, different subnet mask

    Scooby schreef:
    > "James Carlson" wrote in message
    > news:xoavvee99lq1.fsf@sun.com...
    >> The Unlord writes:
    >>> Is there any case where it would be useful to give 2 hosts in the same
    >>> network (and subnet) a different subnet mask? I don't think there is,

    >> The only case I can think of is with bridging by way of proxy ARP.
    >> It's an extraordinarily lame thing to do, very hard to get right, and
    >> falls apart when subjected to broadcast traffic.
    >>
    >>> Host1: 172.16.16.1/20
    >>> Host2: 172.16.16.2/24

    >> Ick. Get out the clue stick.
    >>
    >> --
    >> James Carlson, Solaris Networking
    >> 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

    >
    > Agreed. I'd really love to hear the response when they are asked why they
    > did it this way.
    >
    > Jim
    >
    >


    So, basically: if you can, avoid at all costs!
    Thanks a lot!

  7. Re: Same Subnet, different subnet mask

    On 31 May 2007, The Unlord wrote:

    > Is there any case where it would be useful to give 2 hosts in the
    > same network (and subnet) a different subnet mask? I don't think
    > there is, but one of our customers seems to be doing it and
    > complains about intermittent communication problems (which would
    > seem logical to me), but before we tell them their completely
    > messing things up that way (I don't know exactly how their network
    > is configured), I want to be sure.


    The case you describe looks odd, but I do something vaguely similar
    on the network at work. The subnet for all "normal" machines is
    192.168.1.0/24. But we have a large number of devices that come with
    a default static IP of 192.168.250.250 with a 255.255.0.0 subnet
    mask. We can put them on the network for testing without having to
    reconfigure them by giving the 2 "normal" machines that need to
    communicate with them a static route to 192.168.250.0/24 and just
    leaving the devices with their default settings. Works fine. Of
    course we could have switched everything to a /16 subnet, but we have
    a lot of statically configured machines and this seemed easier at the
    time ;-)

    Dave

    --
    D.a.v.i.d T.i.k.t.i.n
    t.i.k.t.i.n [at] a.d.v.a.n.c.e.d.r.e.l.a.y [dot] c.o.m

  8. Re: Same Subnet, different subnet mask

    On Thu, 31 May 2007 09:16:54 -0400, James Carlson wrote:

    > The Unlord writes:
    >> Is there any case where it would be useful to give 2 hosts in the same
    >> network (and subnet) a different subnet mask? I don't think there is,

    >
    > The only case I can think of is with bridging by way of proxy ARP. It's
    > an extraordinarily lame thing to do, very hard to get right, and falls
    > apart when subjected to broadcast traffic.


    There are other cases, although these are also an extraordinarily lame
    thing to do, very hard to get right, and more or less falls apart when
    subjected to broadcast traffic.

    Point in case, device does not support bonding or any other sane way to
    get two Ethernet cards connected in some sort of failover setup. In that
    case connect both network cards to the LAN, configure one for the lower
    half of the subnet, and the other to the upper half of the subnet.
    Obviously you need a gateway with addresses in both halves and also
    obviously you need some form of failover to try the second address when
    the first doesn't work.

    Yes, it sucks, it's a hack, it's insane. However it does work, we have
    such a setup and it has been working for years.

    >> Host1: 172.16.16.1/20
    >> Host2: 172.16.16.2/24

    >
    > Ick. Get out the clue stick.


    Right.

    M4

  9. Re: Same Subnet, different subnet mask

    In article <465e7eee$0$14239$ba620e4c@news.skynet.be>,
    The Unlord wrote:

    > Hi,
    >
    > Is there any case where it would be useful to give 2 hosts in the same
    > network (and subnet) a different subnet mask? I don't think there is,
    > but one of our customers seems to be doing it and complains about
    > intermittent communication problems (which would seem logical to me),
    > but before we tell them their completely messing things up that way (I
    > don't know exactly how their network is configured), I want to be sure.


    The only excuse I can think of is if some of the hosts are running
    archaic IP implementations that don't support subnet masks, so they
    always use the classful networks.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  10. Re: Same Subnet, different subnet mask

    On Thu, 31 May 2007 23:17:18 +0200, Martijn Lievaart wrote:

    > Yes, it sucks, it's a hack, it's insane. However it does work, we have
    > such a setup and it has been working for years.


    Oh and for the record, I did not set it up like this. In fact I've been
    ranting for two years now that we need to get this insane setup into some
    semblance of sanity.

    M4

  11. Re: Same Subnet, different subnet mask

    "The Unlord" wrote in message
    news:465ed176$0$13864$ba620e4c@news.skynet.be...
    > Scooby schreef:
    > > "James Carlson" wrote in message
    > > news:xoavvee99lq1.fsf@sun.com...
    > >> The Unlord writes:
    > >>> Is there any case where it would be useful to give 2 hosts in the same
    > >>> network (and subnet) a different subnet mask? I don't think there is,
    > >> The only case I can think of is with bridging by way of proxy ARP.
    > >> It's an extraordinarily lame thing to do, very hard to get right, and
    > >> falls apart when subjected to broadcast traffic.
    > >>
    > >>> Host1: 172.16.16.1/20
    > >>> Host2: 172.16.16.2/24
    > >> Ick. Get out the clue stick.
    > >>
    > >> --
    > >> James Carlson, Solaris Networking


    > >> 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
    > >
    > > Agreed. I'd really love to hear the response when they are asked why

    they
    > > did it this way.
    > >
    > > Jim
    > >
    > >

    >
    > So, basically: if you can, avoid at all costs!


    Agreed - bad idea.

    But - often needed if you have a big network and need to migrate between
    network structures - eg taking a bridged flat campus network or MAN and
    converting to routed.

    different masks on the same wire and using proxy ARP to fix up the resulting
    mess during migration has converted impossible changes into only difficult
    ones several times for me.

    Luckily, we seem to be running out of big bridged networks....

    > Thanks a lot!

    --
    Regards

    stephen_hope@xyzworld.com - replace xyz with ntl



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