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Re: default gateway on different subnet

On Nov 15, 2008, at 7:02 AM, Johnny Billquist wrote:

James Chacon skrev:
On Nov 14, 2008, at 5:30 PM, Rhialto wrote:
On Fri 14 Nov 2008 at 06:26:07 +0100, Michael Kell Jensen wrote:
Well i dont understand why some say it is broken apparently some of my
network guys, friends do it all the time.

Well the reasoning is this. I tried to find it in the original RFCs
which define the Internet Protocol, but presumably the authors found it so obvious that there was no need to mention it explicitly; at least I couldn't find it so far. The closest I could find it was a passage in
RFC 950, about subnetting, which I will quote below, but it uses the
weasel-word "Generally" instead of stating that it is the only thinkable

That's because it's not specifying an implementation but just showing possible logic to do so. Nothing in that RFC disallows an interface "knowing" it can directly connect to more than one network out a given interface.

True. It is perfectly fine to configure an interface with two addresses.

However, that was not the scenario described in the original question.

To quote rfc 1122:

    3.3.1  Routing Outbound Datagrams

        The IP layer chooses the correct next hop for each datagram it
        sends.  If the destination is on a connected network, the
        datagram is sent directly to the destination host; otherwise,
        it has to be routed to a gateway on a connected network.

Note the operative word "connected network".

Show a standard requiring "connected network" to mean I have to have an actual interface + IP configured for that network vs just physically being able to see it in some way from the host. I don't believe such a specification exists (that I could find) thereby allowing implementations to easily allow this sort of thing at their choosing.

All of this comes down to implementation details which is a fairly wide open framework to work within.


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