Re: ISO ES to IS Info Sought

Jerry F Aguirre (jerry@oliveb.ATC.OLIVETTI.COM)
Thu, 19 Mar 87 13:31:11 PST

Newsgroups: mod.protocols.tcp-ip
Subject: Connecting two ethernet segments
References: <>
Reply-To: Reply-To: jerry@oliveb.UUCP (Jerry F Aguirre)
Distribution: world
Organization: Olivetti ATC; Cupertino, Ca

In article <> (Charles Hedrick) writes:
>Let us suppose that you have the following configuration:
>--------- network 192.1.1
> |
> router, with addresses and
> |
>--------- network 192.1.2
> |
> router, with addresses and
> |
>--------- network 192.1.3
>Now suppose you have a 4.2 machine on network 192.1.2, whose own
>address is Here is the way I would set up routing.

If this is referring to the article about the Bridge GS/3-M then it is
wrong. Unlike protocol dependent routers the GS/3-M routes at the
ethernet address level. It doesn't know TCP/IP from Decnet and therefor
doesn't have an IP network address. Thus you can not specify it in a
routing table.

Logically the Bridge GS/3-M acts as a repeater allowing you to extend
an ethernet beyond the single segment limits. The difference is that it
is a selective repeater so it filters out ethernet addresses that don't
need to go to the other segment. (It also allows the other segment to
be a LOT further away than a standard repeater would.)

Segments connected by a repeater are logically on the same network. I
spoke to a Bridge representative about this and he said that we would
have to re-address the two networks we were planning to connect so that
they all had the same network number. He suggested going to a type A
address instead of a type C to allow more than 254 hosts/network.

Is this a real requirement? Ignoring the Bridge product, suppose that
two departments each set up independent ethernets. They properly
register their IP addresses so there is no conflict. Later they decide
to attach the two segments together either directly or thru a repeater.

This seems a common enough occurrence to me. If TCP/IP requires that
they renumber all their hosts then this sounds like a significant
flaw in the addressing and routing scheme.

Would it be enough to specify routing to the network via itself? There
is an local entry automatically added that is equivalent to:

        route add net mynet myhostname

Would something like:

        route add net othernet myhostname

be enough to tell it that "othernet" is actually available via the same
interface? Someone out there must have had to connect two previously
independent ethernet segments.

                                        Jerry Aguirre
                                        Systems Administration
                                        Olivetti ATC

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