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From: tsuchiya@THUMPER.BELLCORE.COM (Paul Tsuchiya)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Can subnets be separated by another net?
Message-ID: <9007101422.AA01680@chiya.bellcore.com>
Date: 10 Jul 90 14:22:26 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 48

> .............. 
> The most common case where clients ask about disjoint subnets, is where
> an enterprise is geographically disjoint (say offices in Los Angeles,
> Denver and Boston) and wants to attach each office separately to the
> Internet, while assigning all host addresses out of the same class B
> network number. This of course is utterly undesirable (would OSPF allow
> it to be set up at all ?) and contravenes all the intentions for which
> subnets were invented.
> 
> Thus, for "commercial use" the simple, practical and almost true answer
> is that disjoint subnets are not allowed.
> -- 
> / Lars Poulsen, SMTS Software Engineer
>   CMC Rockwell  lars@CMC.COM
> 

Hmmm.  Why is it <of course> utterly undesirable that one organization
in locations separated by the Internet not split up a Class B address?
The only reason I can think of is that Internet routers are incapable of
looking at subnet parts of the address--in other words, our Internet
routers (or routing protocols) are inadequate.  The thing just simply
wouldn't work.

However, I don't necessarily see something inherently bad about this.
I mean, given today's address structure (net.subnet.host), what are the
alternatives?

First, the organization could have multiple class C addresses? However,
this puts a load equal to subnetted class B addresses on the Internet.
Each router must maintain one routing table entry for each location.
But, it is highly unlikely that a single class C address will suffice
for a location, so the organization probably needs a different class B
address for every location.  We still haven't decreased the load on
the internet (one routing table entry for each location), but we have
managed to make bad use of our address space.

In other words, by not having protocols that allow the Internet routers
to look into the subnet, we have NOT decreased the amount of routing
overhead, but we HAVE used our limited address space poorly.

Does BGP have masks?  If so, it could look at the subnet part.  This
would at least allow use to use addresses a little more efficiently.
However, a more general, multi-level hierarchical address scheme 
coupled with an efficient address assignment scheme is what's needed.
Hear my ideas on this at the UCB IETF, or if you can't be there,
ask me and I'll send you a paper on the topic.

PT