Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!ncar!gatech!mcnc!decvax!ucbvax!HOGG.CC.UOREGON.EDU!jqj
From: jqj@HOGG.CC.UOREGON.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  mac-level bridges and internet addresses
Message-ID: <8804122257.AA13640@hogg.cc.uoregon.edu>
Date: 12 Apr 88 22:57:11 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 26

>We have an Ethernet here with hosts on both
>18.0.0.0 and 128.30.0.0 without (many) problems.

One must be very careful when running multiple IP networks on the same
cable.  For example, it makes it much more likely that a misbehaving
gateway will cause a broadcast storm aka "meltdown."  Typical example:
suppose that 128.30.0.0 is 8-8 subnetted, and that subnet 128.30.3.0 is
on the shared cable.  Suppose also that both IP networks have gateways to
the arpanet or something.

Suppose some host on 128.30.3.0 sends an IP broadcast (as an Ethernet
broadcast) to its subnet broadcast address, say to 128.30.3.255.  A
gateway between 18.0.0.0 and the arpanet, say 18.0.0.1 (but NOT also
configured to be on 128.30.3.0), receives the packet, notes "this is a
packet for host 3.255 on net 128.30.0.0" (please observe that since
this gateway does not itself have any interfaces on 128.30.0.0, it
doesn't know what the subnet mask of that network is and hence can't
recognize this as a subnet broadcast), sends it to a gateway on
128.30.3.0, who explodes it onto the cable.  The first gateway gets
another copy, forwards that packet, etc.  As Chuck Hedrick noted in a
recent message to tcp-ip, it is essential that gateways pay attention
to whether the packet arrived as a media broadcast; unfortunately, many
gateways (e.g.  4.3BSD) do not.  A fortiori, unless you're willing to
also have gateways drop ALL packets sent as Ethernet broadcasts (which
I am not), or are willing to insist that any gateway on a physical
network know about all the nets on that cable (which I don't see how
you can do), you must also forbid topologies such as the one above.