Path: utzoo!attcan!uunet!husc6!mailrus!ames!pasteur!ucbvax!proteon.COM!jas
From: jas@proteon.COM (John A. Shriver)
Newsgroups: comp.protocols.tcp-ip
Subject: Subnet to Subnet routing
Message-ID: <8806141502.AA04498@sonny.proteon.com>
Date: 14 Jun 88 15:02:23 GMT
References: <8806132340.AA01240@monk.proteon.com>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 20

Subnets of a given subnet must be contiguous.  The definition of
contiguous is that all subnets of the net are reachable via routes
internal to that network.  It's in RFC 950 and RFC 1009.

The whole point of subnets is to hide the internal structure of a
network from all other networks.  In your example, the Class C network
has been split, and is not contiguous, since traffic between the
subnets must cross the Class B network.  Since hosts on the Class B
network are routing to the Class C network on a network (not subnet)
basis, how will they know whether to send traffic for SUN3 via SUN1 or
SUN2? 

Looking at your configuration, subnet your Class B network, using from
4 to 8 subnet bits.  Make each Ethernet one subnet.  Subnetting Class
C networks is not typical.

If there are some hosts on the backbone Ethernet that don't know from
subnets, you may be able to fake them out using proxy ARP subnet
routing.  However, they had better age their ARP cache if they do
this.