Path: utzoo!attcan!uunet!cs.utexas.edu!rutgers!mailrus!ames!sgi!wdl1!holmes
From: holmes@wdl1.UUCP (Randy D Holmes)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Dividing Class B number in subnets, Routing Problem
Message-ID: <3950008@wdl1.UUCP>
Date: 26 May 89 15:06:47 GMT
References: <159@pollux.kulcs.uucp>
Lines: 65



   Stephan,
      I understand your problem, we were faced with it a while back.
      Based on our experience I recommend you avoid using different 
      sized subnets if at all possible. If you haven't already read
      RFC950 "Internet Standard Subnetting Procedure" you should.
      Here is an important point stated in the RFC

     ...
        For example, the Internet address might be interpreted as:
 
            <network-number><subnet-number><host-number>
 
         where the <network-number> field is as defined by IP [3], the
         <host-number> field is at least 1-bit wide, and the width of the
         <subnet-number> field is constant for a given network.  No further
         structure is required for the <subnet-number> or <host-number>
         fields.  If the width of the <subnet-number> field is zero, then
         the network is not subnetted (i.e., the interpretation of [3] is
         used).
     ...

     In other words what you want to do is non-standard.

     Having said that, if you still want to go ahead, heres what we 
     did. Our class B network uses 3-bit subnetting at the top level,
     2 of those subnets are further subnetted to 8-bit subnets. Because
     of the RFC950 requirement to have subnet widths fixed for a given
     network, we were NOT able to do this using one gateway (we use
     cisco gateways). We have one gate way configured to do 3-bit
     subnetting, and one to do 8-bit subnetting, they are separated by
     a serial link and there are more gateways on both sides of this link.
     Immediate problems arise if you try to do dynamic routing. All the
     gateways on the 8 bit side send routing info for all the 8-bit 
     subnets they know about, but the 3-bit gateway just sees this as
     several routes to 2 3-bit subnets, often with the routing information
     conflicting. i.e. an 8-bit subnet which is 2 hops from the 3-bit gateway
     looks like the SAME subnet which is one hop away, and both routes use the
     same first hop. Also, and I'm a little less clear on this one, the
     3-bit gateway and the 8-bit gateway seem to argue over who has the correct
     subnet mask, and eventually ignore each other. Going the other way,
     hosts 128.5.192.2, and 128.5.193.2 are both on 3-bit subnet 128.5.192,
     however all of the 8-bit subnetted gateways think these are separate
     8-bit subnets, and DON'T know how to get to 128.5.193.2. 
     Our final solution was to turn off the dynamic routing on the 3-bit 
     gateway, and make all of its routes static. We also had to enter 32
     static  routes on the 8-bit gateway for EACH 3-bit subnet in use on the
     other side of the 3-bit subnet gateway.

     The bottom line is, we are working, and working quite well. But
     because we are doing things in a non-standard way we have an
     administrative headache on our hands, and a large portion of
     our down time has been attributed to this problem. Also we only
     have 2 sizes of subnets, you seem to propose at least 3. I should
     also mention that steps are being taken to eliminate this kludge in
     our network.

     I am sending this message not as a solution to your problem, but
     as an argument against doing what you propose.

     I hope this helps.


     Randy    holmes@wdl1.fac.ford.com