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Re: subnets & 2 NICS in a mashine

On Friday 10 November 2000 14:48, Robert Guthrie wrote:
> As you may have figured out, having 2 NICs on the same subnet would not do
> anything for you.  The computer with 2 NICs would either recieve duplicate
> packets, and have to do double the work (forwarding duplicate packets), or
> it would have to ignore one of the nics.
> It doesn't HAVE to be this way; someone could re-write the tcp/ip protocol
> if they wanted two NICs on the same subnet, but there's no point.

It occurs to me that you might be thinking "but what if I want one computer 
to have two nics with two different IPs on the same subnet?", which is 
something that will not cause you problems (the tcp/ip protocol has no 
problem with this).  However, while anybody could send you data on either of 
the ip addresses asssigned to that machine (like and, with netmask of, the machine itself would only 
use one (probably which ever NIC was assigned to the eth0 device), ignoring 
the other NIC for out-going data.

In addition, this would have zero effect on network traffic (acutally, it 
would increase broadcast traffic on that subnet, as it would respond once for 
each NIC), and so there's no practical advantage to have two NICs in one 
computer if they're going to be on the same wire.

If these two NICs are going to be on different wires, then they have have 
different subnets, or the computer won't know which NIC to trasmit on.

If you could give an example (including ascii diagrams) of what you're trying 
to accomplish, and and explanation of what advantage you hope to get from 
that setup, maybe we can get closer to understanding what you really want to 
do.  I don't think I'm really qualified to keep attempting to teach you the 
OSI* networking model, and I don't think I can make this any clearer with my 
limited skills.

*Is that the right name? I learned that thing 8 years ago, and have never 
needed it since.
Did you know that if you play a Windows 2000 cd backwards, you 
will hear the voice of Satan?

That's nothing!  If you play it forward, it'll install Windows 2000.

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