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Re: default gateway on different subnet

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.

Routing generally falls into 2 classes:

1. Stuff I can directly connect to (usually by being able to ARP, get a MAC and form a packet to drop onto the wire using ethernet for instance, but other physical mediums would be similar)

2. Stuff I need a gateway in order to send packets. Here I form packets which go to the gateway but have another IP as their destination. i.e. the physical layer cannot see the destination IP and needs something in between to get the packets there.

There is nothing conceptually wrong with saying "I'm connected to a cloud which has these N networks directly accessible and my local IP is on one of them" and some way to then specify what those N networks are. In this case then there would be no routing since the machine would be able to directly see the other hosts. Folks who believe there is some hard and fast rule which requires interfaces to have IP's configured for each of these networks I challenge to show me any specification which requires this vs simply allowing additional netmasks to be added to a given interface. i.e. it's all an implementation detail for the most part and there's a lot of allowed flexibility here.

This all presumes the other hosts on this cloud of multiple networks are all configured in some reasonable way to be able to get packets back to you as well of course.


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