setting IP address manually

[Bruno Wolff III]

On Mon, Oct 24, 2005 at 23:27:55 -0500,
  Mike McGrath <mmcgrath at iesabroad.org> wrote:
>  
> 
> > -----Original Message-----
> > From: fedora-list-bounces at redhat.com 
> > [mailto:fedora-list-bounces at redhat.com] On Behalf Of Bruno Wolff III
> > Sent: Monday, October 24, 2005 9:51 PM
> > To: Tim
> > Cc: For users of Fedora Core releases
> > Subject: Re: setting IP address manually
> > 
> > On Mon, Oct 24, 2005 at 11:59:01 +0930,
> >   Tim <ignored_mailbox at yahoo.com.au> 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.
> > 
> > I don't think that is correct. Ethernet interfaces have mac 
> > addresses that identify them uniquely.
> > As far as routing goes, that is based on the destination 
> > address, not the source address. It is more common that you 
> > want the source address to match the outgoing interface so 
> > that return packets are routed more efficiently, but there 
> > are situations where you want to use the same source address 
> > no matter which outgoing interface you use.
> 
> This is confusing the data link and network layers.  Mac addresses have
> nothing to do with IP routing.  And you could have one mac address that
> has multiple IP addresses.  (For example an aliased interface, like
> eth0:0)  At no point in time should two seperate machines have the same
> IP address.  There's goofy ways you could force it to work by manually
> editing the ARP tables on both machines but I digress.

I wasn't advocating using the same IP address on different hosts. The IP
address of an interface is the default IP address to use for the source address
for outgoing packets on that interface. Having the same IP address on different
interfaces of the same host isn't itself a problem. (Though this might result
in inefficient return paths being used in some situations.) Which interface
packets are sent to is determined by the destination IP address and your
routing tables.