Two subnets thru same wire

[Ed Greshko]

Shiraz Baig wrote:
> Sir,
> Mr Ed Greshko has reminded me about academic
> questions. I agree with him. But it is not purely an
> academic question, as would be clear from following
> real life situation of my office.
> BTW, I have read, everything about ARP, MAC address
> and how things work on wire. But I thought, I must
> also also ask experts.
> 
> Here is the configuration that we have in our office.
> This configuration is also working.
>                                   
>     Switch
>  _______________
>  |  |  |  |    |
>  |  |  |  |     ---- DSL Router  ---- Internet
>  |  |  |  |         203.147.175.12
>  |  |  |  |
>  |  |  |  ----------Mail Server (multihomed)
>  |  |  |            203.147.175.13
>  |  |  |            192.168.10.3
>  |  |  |
>  |  |  -------- 192.168.10.4
>  |  ----------- 192.168.10.5
>  ---------------- Hub & computers of address  
>                    192.168.10.*
> 
> You see, there are two class C networks. One is that
> of 203.147.175.0
> The other is 12.168.10.0. Both these networks are
> passing thru same switch. 
> 
> I just wanted to be sure that there is nothing wrong
> with this configuration. So, I would request your
> comments please.

Things keep changing.  No longer is this an "academic exercise" it seems 
now to be a "free consulting" exercise.  :-)

Your question shouldn't be "can I do this" but "even if I could, should 
I do this".

As a practical matter you don't want to do what you've diagramed.  At 
the very least you'd be better served by:

    Switch
  _______________
           |    |
           |     ---- DSL Router  ---- Internet
           |         203.147.175.12
           |
           ----------203.147.175.13
                            |
                            |
                   Mail Server & Firewall (multihomed)
                            |
                            |
                     192.168.10.3
                            |
                            |
                 ------------
    Switch       |
  _______________|____
  |  |  |
  |  |  |
  |  |  |
  |  |  |
  |  |  |
  |  |  |
  |  |  |
  |  |  |
  |  |  -------- 192.168.10.4
  |  ----------- 192.168.10.5
  ---------------- Hub & computers of address
                    192.168.10.*


Kindly excuse the poor ASCII art.  I was never very good at it.

-- 
"A common mistake that people make when trying to design something
completely foolproof was to underestimate the ingenuity of complete
fools."

--Ford Prefect in "Mostly Harmless".