Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!pasteur!agate!saturn!eshop
From: eshop@saturn.ucsc.edu (Jim Warner)
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  Echo on undefined subnet mystery (solution)
Keywords: Subnet default route
Message-ID: <5585@saturn.ucsc.edu>
Date: 26 Nov 88 05:10:43 GMT
References: <8811251901.AA04669@sneezy.lanl.gov>
Reply-To: eshop@saturn.ucsc.edu (Jim Warner)
Organization: University of California, Santa Cruz
Lines: 24

In article <8811251901.AA04669@sneezy.lanl.gov> cpw%sneezy@LANL.GOV (C. Philip Wood) writes:
>	   Note, it does not send a redirect cause its forwarding on a
>	   default route.

I am not aware of such a rule.  Is it possible that your gateway B
is a Sun?  Our Suns *never* send redirects.  I think such a rule would
be a bad idea.  Hosts should be dumb and gateways should be smart, and
all that stuff.  How's a dumb host supposed to find out where to send
packets if not for redirects?

I set up a situation similar to yours by pinging from Stanford to
a nonexistant Santa Cruz subnet.  This is against the flow of the
default path.  All I get back is a single destination unreachable
message.  It appears that Proteon routers do not share your problem.

To route the packet from A, B should first apply the class C net mask,
ff.ff.ff.00 to the destination address.  It determines this matches one
of its nets.  It then applies the subnet mask for this net, ff.ff.ff.e0
in your case.  It doesn't find a matching subnet route.  If it has a
default subnet route the packet can be forwarded in that direction.
I claim that it should not use the default *NET* route as a sort of
poor substitute for a missing subnet default route.

jim warner