two interfaces one subnet

Skywing Skywing at
Mon May 11 20:31:37 CDT 2009

Yes, similar happens to me all the time with both Windows Server 2008 and Vista with respect to 802.11 putting two interfaces on the same subnet (and LAN segment).  I typically am never the wiser until I notice that a SMB connection had gone over to 802.11 first, because that associated before the wire was plugged in, causing data transfers on that connection to be much slower than on a hardline.

This is something that happens every day to a great many laptop users, I'd wager.  Certainly not by any means up there in "does not work at all, period / incompatible with networking" land, as, at the very least, emperical evidence has a quite different story to say.

- S

-----Original Message-----
From: Chris Adams <cmadams at>
Sent: Monday, May 11, 2009 16:29
To: nanog at <nanog at>
Subject: Re: two interfaces one subnet

Once upon a time, Kevin Oberman <oberman at> said:
> > From: Chris Meidinger <cmeidinger at>
> > For example, eth0 is and eth1 is, nothing like
> > bonding going on. The customers usually have the idea of running one
> > interface for administration and another for production (which is a
> > _good_ idea) but they want to do it in the same subnet (not such a
> > good idea...)
> This will not work right. One interface can be, but any
> added interfaces would need to be /32 (

I don't know which OS(es) you are using, but that's not true in Linux.
I see this all the time at home; if I plug my notebook into the wired
LAN and still have the wireless enabled, both will get an IP (in the
same subnet) from DHCP.  The wired link is the preferred default route
by default, but you can easily set up routes for some networks via the
wireless link.

You can also set up multipath routing to send packets out both links.  I
think you can also use IP policy routing to control the choice of
outbound interface by rule (e.g. based on source address).

Chris Adams <cmadams at>
Systems and Network Administrator - HiWAAY Internet Services
I don't speak for anybody but myself - that's enough trouble.

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