Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!agate!ucbvax!CC.MSSTATE.EDU!peters
From: peters@CC.MSSTATE.EDU (Frank W. Peters)
Newsgroups: comp.protocols.tcp-ip
Subject: Subnets on an unsubnetted network
Message-ID: <8901302314.AA07244@Jester.CC.MsState.Edu>
Date: 30 Jan 89 23:14:30 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 74

Hello,

     A recent posting by Tom Corsetti brought to mind several questions 
which we wrestled with and often left unanswered.

     We, like Mr. Corsetti, have a large un-subnetted class B network.  
Addresses on this network are assigned in a subnetted fashion (that is,
all of the nodes in a given building have the same third octet to allow
for subnetting at a later date).

     In a couple of buildings on campus we have Sun servers with two
ethernet cards and diskless workstations 'behind' them to provide nfs
traffic isolation.  Each network of clients has its own 'subnet' of the
campus class B network.

     We are not exactly sure about how to configure the gateway suns
to allow them to be subnetted (for the clinets on one side) and still
have full connectivity with the unsubnetted campus network. 

     As it now stands we have the following problems:

(1) Broadcast Addresses: As it now stands, the subnetted machines address
    broadcasts to (say) 130.18.64.255 instead of the common address of
    130.18.255.255.  Is there any reason why we can't take advantage of
    the broadcast option of the UNIX ifconfig command to have a subnet mask
    of 255.255.255.0 and a broadcast of 130.18.255.255?  should the broadcast
    be the same on BOTH sides of the gateways?  Does it matter if its the
    same?  That is, can the broadcast be x.x.255.255 on the campus side and
    x.x.y.255 on the client side?
 
    As it now stands, every time a subnetted machine on the 130.18.64.0 net
    (which is really part of the unsubnetted campus net) sends a broadcast 
    the subnetted machines on (say) the 130.18.48.0 net (which is also part
    of the same campus net but, because of subnetting, THINKS its on a
    seperate subnet) don't recognize the broadcast and attempt to arp the
    130.18.64.255 broadcast address.

(2) Routing (1):  Given a gateway sun that is on the unsubnetted campus backbone
    but that THINKS it is on a subnetted network, how do we convince it that
    it can reach a machine directly that is not on its 'subnet'?  That is,
    how do we convince a subnetted gateway machine on the 130.18.64.0 portion
    of the backbone that it can directly reach a machine on the 130.18.48.0
    portion?  As it is now configured all packets of this sort are routed to
    our proteon routed...which turns right around and sends it back out the
    same interface.  Gross huh?

(3) Routing (2):  Are there any options besides proxy arp to allow the gateway 
    suns to grab packets off of the backbone and routing them through their
    client sides?  This is basically the common problem of an unsubnetted
    machine sending a packet to a machine that is not REALLY on the same wire.
    We have a public domain proxyarpd package for suns but can't get it to
    compile on a sun 4.

(4) Sorta-Subnetting:  This is a possibility that may solve all of our 
    problems if I only understood it.  I note that (on a Sun at any rate) it
    is possible to assign subnet masks (and subnetting alltogether) on an
    interface basis.  What would it mean to have a gateway with two subnets
    for the same network on two different interfaces?  Would it help?  Would
    it WORK? How about one interface subnetted and one not (thats pretty much
    what we logically want)?

     Any comments or suggestions would be welcome.  Please don't suggest 
subnetting the campus backbone though.  We just don't have the budget for 
the routers yet.

                    Thanks
                    Frank Peters

=====================================================================
Systems Programmer                 |   Mississippi State University
Phone:    (601) 325-2942           |   Computing Center and Services
Internet:  peters@CC.MsState.Edu   |   Post Office Drawer CC
BITNET:    PETERS@MSSTATE.BITNET   |   Mississippi State, MS.  39759
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