setting IP address manually

[Jeff Vian]

On Mon, 2005-10-24 at 11:59 +0930, Tim wrote:
> Mike McGrath
> 
> >> Though I believe the issue you may be having is that you're trying to
> >> assign the same IP address to both interfaces.  They should have a
> >> different IP address (and ideally be on a different network)  for
> >> example, eth0: 192.168.1.250,  eth1: 192.168.2.250.
> 
> Bruno Wolff III:
> 
> > No that isn't it. You can use the same IP address on different interfaces.
> 
> That's asking for problems.  IP addresses relate to interfaces, not
> machines.  Each interface should have an unique address.  You'd be
> relying on your system trying to sort out problems for you with the same
> IP on different interfaces, and that's never a good thing.
> 
> > The message about an IP address already in use, means some other system on
> > the same network is using that address.
> 
> How's it going to do that, then?  My computer has no idea about another
> computer on my network unless it talks to it, and they don't do that
> unless I deliberately try something between the two of them.
> 
Not quite true.  The NIC identifies other devices by mac address and the
TCP protocols map mac addresses to IP addresses as soon as the interface
is configured.   A check is made to find out if the address is valid on
the network when an attempt is made to bring the interface up.  If it
finds another with the same address it refuses to activate the
interface.

You do not have to actively tell it to do so, it is part of the
configuration protocol and that behavior is seen on all OSes I have ever
worked with (except maybe the antique DOS). For sure all modern TCP
stacks make that check.


> I *can* set two PCs on a network with the same IP address, it's an
> accident that happens time and time again on any network that has
> visiting PCs.  Of course I'm going to have lots of problems, but nothing
> stops me from doing so in the first place.
> 
> -- 
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> I read messages from the public lists.
>