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From: medin@NSIPO.NASA.GOV ("Milo S. Medin", NASA ARC NSI Project Office)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Can subnets be separated by another net?
Message-ID: <9007080605.AA00749@cincsac.arc.nasa.gov>
Date: 7 Jul 90 22:05:35 GMT
References: <2346.2696efa3@csc.anu.oz>
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 107


	 In article <1867@trlluna.trl.oz>, m.andrews@trl.oz.au (Murray Andrews) writ
	es:
	 > I have a basic question about subnet routing that has probably been asked
	 > many times but I can't locate an answer in any article sitting around
	 > here so ....
	 > 
	 > Is it possible to route between subnets of a class B address when the
	 > subnets are separated by another network?
	 
	 The short answer is no.

Well, that's not quite right.  This depends on your routing protocol and how your
routers actually forward packets.
	  
	 > For example, given the following topology:
	 > 
	 >           +---------+
	 >           | Host C1 |
	 >           +---------+ 192.9.200.5
	 >                |
	 >        --------+-----+----- Net 192.9.200 ----------+---------
	 >                      |                              |
	 >                +----------+ 192.9.200.1        +----------+ 192.9.200.2
	 >                | Gate GB1 |                    | Gate GB2 |
	 >                +----------+ 137.147.1.10       +----------+ 137.147.2.20
	 >                     |                              |
	 >        ------+------+-------               --------+---+------
	 >              |                                         |
	 >         +----------+ 137.147.1.11                 +----------+ 137.147.2.
	21
	 >         | Host B11 |                              | Host B21 |
	 >         +----------+                              +----------+
	 > 
	 > 
	 >        ^     Subnet 1 of 137.147    ^     ^       Subnet 2 of 137.147    
	 ^
	 >        |____________________________|     |______________________________
	_|
	 > 
	 > Host C1, and gateways GB1 and GB2 all connect to the one network - in thi
	s
	 > example a class C network with number 192.9.200 (don't worry - we are not
	 > actually using this number).
	 > 
	 > Gateways GB1 and GB2 are gateways to 2 subnets of the the class B
	 > network 137.147 with subnet mask 255.255.255.0. There is no connection
	 > between the two subnets except via 192.9.200.
	 > 
	 > The question is does this work?
	 
	 No. The reasons is that the Gateways will only advertise a route to 137.147
	.0.0
	 (not to a particular subnet of that network) over the 192.9.200.0 subnet th
	ey
	 are connected too.
	 
Again, this depends on the routing protocol and routers.  It is certainly easy 
enough to configure the OSPF protocol to make this work.  In fact, the NASA
Science Internet network (built of Proteon p4200 routers) does exactly this.
We have 2 NSI routers seperated by a class C network, and both routers can
deal with parts of 128.161 on both "sides" of the class C net.  This is
a consequence of variable length subnet support, and how OSPF areas 
are configured.  


	.
	.
	.

	 The only way it *might* work is to give the interfaces that connect the two
	 gateways a second IP address (in this case from another subnet of 137.147) 
	 and use a bit of static routing in the hosts. Two subnets (from different n
	et
	 numbers would then share the same physical network).
	 
	 cisco routers support secondary interface addresses but will never generate
	 an
	 IP packet with the secondary IP address - which is why you might have to us
	e
	 some static routing. I've done something like this at Macquarie University 
	to
	 support CSIRO's links into that campus, but at that site it's a case of two
	 networks on the same cable, not a partititioned subnets (which is illegal
	 according to the RFC),

This is a real kludge, and any hosts on the net in the middle may cause problems 
because of improper handling of broadcast packets.  As I said, with the right 
routing protocol, this situation can work.  We actually discovered this 
accidentally one day, when we configured things in such a way that this 
behavior resulted, and then decided that it wasn't supposed to work!  After
thinking things through however, it was clear this was a topology the system
could support.

Now, I wouldn't go off recommending this approach to people, but it certainly can
be useful at times, especially during transitions, and for other reasons too.
So, Murray, the answer to your question is yes, given you have OSPF routers 
involved.  Otherwise, I think the kludges you need would be pretty ugly and
not work well.

OSPF is brought to you by the IETF, and is documented in RFC 1131.  Expect to
see a multivendor demo at InterOp this fall.  Ask for it by name, accept no
substitutes!  


							Thanks,
							   Milo