Path: utzoo!mnetor!uunet!husc6!hao!ames!ucbcad!ucbvax!AI.AI.MIT.EDU!JBVB
From: JBVB@AI.AI.MIT.EDU ("James B. VanBokkelen")
Newsgroups: comp.protocols.tcp-ip
Subject: Re: More than one IP (sub)network on one ethernet cable
Message-ID: <302655.871221.JBVB@AI.AI.MIT.EDU>
Date: 22 Dec 87 04:42:56 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 37

Ok.  There are a number of situations where this configuration (more than
one net or subnet on a single broadcast medium) seems to bring some benefit.

However, it goes against a fairly fundamental paradigm of RFC 791 and
RFC 950, the use of the network (and subnet) mask to make a simple routing
decision.   My present code, and its installation, configuration, debugging
& documentation all benefit from this.

I assume that if you're going to break the paradigm, you want to go whole
hog and have different classes of networks, and different numbers of subnet
bits, all on the same cable?  So the configuration would have to look like:

 mask 255.255.224.0	net 128.4.32.0	local	; class B, 3 subnet bits

 mask 255.255.0.0	net 18.10.0.0	local	; Class A, 8 subnet bits

 mask 255.255.255.0	net 192.9.1.0	local	; Class C, 0 subnet bits

Three nets wouldn't be enough - I'd better design for 8, or 16.  The list
gets scanned for every miss on the ARP cache.  Not too expensive.  However,
the configuration would have to be set, correctly, on every host on the net.
Is it still safe to assume that everything which is not on the list should
go through a default gateway, or do workstations need explicit routes there,
too?

We can write the code, we can document it, we can probably even explain it
to the non-guru (to whom setting up an IP network is already a major
adventure).  Right now, I don't think it is justified.  Maybe once automatic,
central configuration initialization & control is available, maybe once the
Unix manufacturers all get into agreement about the nature of broadcast
packets, maybe once IP Multicast is generally understood, and implementations
are beginning to show up (in DC, Postel warned that this was something
vendors should keep their eyes on).  It looks like a low priority to me. 

Are there people with money to spend that say otherwise?

jbvb