🚀 go-pugleaf

Alan,

   There is just one problem with getting a class C....

> Return-Path: <cisco-request@spot.Colorado.EDU>
> From: Watt-Alan@mickey.ycc.yale.edu ()
> Date: Fri, 21 Jun 91 22:36:13 EDT
> To: hubert@cac.washington.edu
> Subject: Re: backplane backbone, routing or bridging?
> Cc: cisco@spot.Colorado.EDU
> 
> Steve Hubert replied to my posting of 20-Jun-91:
> 
> |On Thu, 20 Jun 91 22:11:40 EDT, Watt-Alan@mickey.ycc.yale.edu wrote:
> |
> |> Subject: Re: backplane backbone, routing or bridging?
> |> To: hubert@cac.washington.edu
> |> cc: cisco@spot.colorado.edu
> |>
> |> ...
> |>
> |
> |> However you do not have to assign a whole subnet to each point-to-point
> |> ethernet.  You can have a single subnet which is shared by all the
> |> point-to-point ethernets on both ends.  Assuming you subnet a class B
> |> on an 8-bit boundary, and the "point-to-point cloud" is subnet 100,
> |> you could define the subnet mask 255.255.255.252 to allow 6 bits
> |> of "sub-subnet" and 2 bits of "host" (0 is reserved, 3 is broadcast,
> |> 1 and 2 are the two endpoints.
> |
> |I don't understand how to do this with cisco equipment.  My understanding is
> |that the cisco has a single netmask for a network.  How do you get the
> |variable length mask idea to work?
> 
> I think my fingers ran amok before my mind was properly engaged.  I have
> actually never tried this and now that I think about it, it is quite
> likely to get you in trouble.  However, I am quite sure that nothing
> stops you from getting yourself a new class C network and subnetting
> it as described above.  This still saves you from wasting unnecessary
> subnets of your class B.

The only possible problem with this comes if you want to put your
class C subnet in between class B segments.

ie:   (Disclamer:  It is late and my brain is tired.  I don't
       guarantee that the addresses below make sense.)

Denver Router                            New York Router
E0: 131.108.19.40                        E0: 131.108.29.47
S0: 192.27.38.249                        192.27.38.250

Note that this is illegal unless there is some other connection
between the two routers that has an address of 131.108.xxx.0.  This is
because discontiguous subnets are illegal.  

Best Regards,

-Nick

> 
> For all that, it may actually work to sub-subnet your class B.  On
> a typical BSD system, datagrams would be output on the proper interface,
> but the routing protocol daemons might get hopelessly confused.  Perhaps
> the same thing would happen on a cisco box (or is it "Cisco box" these
> days?).
> 
> 	- Alan S. Watt
> 	  High Speed Networking, Yale University
> 	  Computing and Information Systems
> 	  Box 2112 Yale Station
> 	  New Haven, CT  06520-2112
> 	  (203) 432-6600 X394
> 	  Watt-Alan@Yale.Edu
> 
> 
> Moral:		Think twice, hit <ENTER> once.
> 
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>