Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!bu-cs!encore!cloud9!jjmhome!cpoint!martillo
From: martillo@cpoint.UUCP (Joacim Martillo)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Subnets on an unsubnetted network
Message-ID: <2026@cpoint.UUCP>
Date: 1 Feb 89 23:54:00 GMT
References: <8901302314.AA07244@Jester.CC.MsState.Edu>
Reply-To: martillo@cpoint.UUCP (Joacim Martillo)
Organization: Clearpoint Research Corp., Hopkinton Mass.
Lines: 55

I am not sure I understand the problem.  Perhaps, I am wrong but
I would consider subnetting a useful administrative kludge when
I have several physical networks but one network number.

With such understanding gateways between two subnets of one
network number should not behave much differently than gateways
between two networks with two different network numbers.  I would
not want my gateway to forward IP broadcast packets from one
network or subnet to the other network or subnet.  That just allows
little sins to turn into big sins.

Now if I have hosts that understand subnets and hosts that do not
understand subnets on the same physical network.  I would expect
the hosts that understand subnets to use the subnet IP broadcast
and hosts that do not understand subnets to use the standard
network IP broadcast.  I would expect the subnet understanding
hosts to understand all IP broadcasts while the hosts which do
not subnet would not understand subnet IP broadcasts.  Such is
life and may be acceptable, otherwise you have to run subnetting
software on all machines.

Now if I understand the case presented, you have gateways which
understand subnetting and on one side of each gateway you have hosts
which understand subnets while on the other side of each gateway you
have hosts which do not understand subnets and there is but one network
for all the hosts which do not understand subnets. 

On the subnetted side, life is simple.  A given host either arps IP 
address on the subnet or sends IP packets to the gateway when the 
destination IP address is not on the same subnet.  

On the unsubnetted side, life is rotten.  Suppose a host which does not
understand subnets wants to send IP packets to a subnetted host.  This
host will assume that the subnetted host is on the same physical network
and will arp it.  Unless the gateway on the subnet host can do proxy
arp, there will be no response to the original arp request and the IP
address will not be resolved.  If the non-subnetted hosts can be
pursuaded to send IP packets with unresolved IP addresses to a gateway,
it would be possible to establish communication but there would be no
obvious basis on which to choose to which gateway to send the packet.  I
suppose you could depend on ICMP redirect to fix this but it seems
rather gross and lots of hosts ignore ICMP redirect so that an IP packet
might always be sent to the wrong gateway first which would then send
the packet to the right gateway.  The bottom line really is that in your
network hosts which do not have a subnet address should still be doing
subnet routing, and until you get the appropriate software you will have
problems. 

As for the gateways, as long as they understand subnet routing properly,
it should be possible for one interface to be attached to a subnetted
physical network with appropriate subnet mask while the other interface
is attached to an unsubnetted physical network. 

By the way, subnetting should be just as possible on a class C network
as on a class B network.