SUMMARY: two network interfaces, one network

From: Steve Ozoa (
Date: Sat Oct 29 1994 - 05:53:28 CDT

My original question was:

>We're in the process of renumbering our internal networks, and to help with the
>transition, we want to set up one of our Sun servers to route between two
>different network numbers, but on the same physical network. Something like
> ----------------------------------- one physical ethernet
> | |
> | |
> | |
> | |
> --------------------
> | |
> | Sun 630 |
> | |
> --------------------
>Currently using SunOS 4.1.3, but we may attempt it with Solaris 2.3 later on.
>Is this possible? If so, exactly how is it done, and what problems are we
>likely to run into?
>We know this isn't a nice thing to do; it's strictly temporary while we get all
>the hosts renumbered.

First, I should apologize for leaving out some information: There are two
network interfaces on the machine in question, and we actually have tried to
simply ifconfig each one with a different network number. The problem we ran
into was with broadcast storms.

The responses pretty much covered the spectrum. One or two people said they've
tried this sort of thing and it isn't worth it; just spend a weekend
renumbering everything at once. We thought of that, but we were trying to
avoid it; we're talking about a hundred or more PC's spread across several
buildings. Plus, this will have to be done on several networks eventually,
though this first one is by far the largest and most painful.

Some people felt it should work if each interface was ifconfig'd with a
different network number. Others said broadcast storms would result due to
both interfaces having the same MAC address (which is no doubt what happened to

A couple of people suggested using just one interface and adding a static route
to the new network number (with 'route add...'). I don't know why I didn't
think of that, I guess my brain was on vacation.

Another suggestion was to try it with two dual-homed machines, i.e.,

        |Net one | Net two
        | |
  +----------+ +-----------+
  | | | |
  | | | |
  +----------+ +-----------+
        | Net three |

This is untested, but it seems like an excellent idea. The only problem is
that it requires an additional network card, which means some juggling of

We haven't gotten around to trying any of these yet, but we may be able to
avoid it; as part of the same project we'll shortly have a Cisco router
inhouse, which explicitly supports routing between two networks on one
interface. We should be able to use it temporarily for this purpose.

If anyone ends up in this situation, I'd suggest trying things in this order:
a static route over one interface
two dual-homed machines
bite the bullet and do everything at once.

Thanks to:

and anyone else I may have left out.

Steve Ozoa
CAD System Administrator 408-436-4292
ATMEL Corporation fax 408-436-4200 pager 408-233-5966

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