Re: default gateway on different subnet

[Michael Kell Jensen <mkj%micrun.net@localhost>]

Greg A. Woods; Planix, Inc. wrote:
> An additional netmask is not really sufficient information with which to 
> specify another IP network, at least not for the whole picture to work.
>
> Even if the network number portion matches bit for bit up to the longest 
> netmask doesn't mean you can get away with just one host address when it 
> comes to trying to do routing.  Networks are specified by a network 
> number and a netmask.  Just because you can do a bit-for-bit matching of 
> two different masked network numbers and see that they match a given 
> host IP doesn't mean you can easily create a mechanism which allows for 
> use of a host IP and a bunch of netmasks to specify a cloud of 
> interconnected networks, at least not on top of a system which currently 
> is designed to specify independent networks (whether they're connected 
> to the same interface or not).
>
> In any case you don't really have a proper cloud of networks in the way 
> you've described the addresses being used.  You have one network and 
> some hosts have artificially restricted their view of that network by 
> specifying a narrower netmask than they could.  I'm not sure how this 
> affects ARP, but at least from their IP routing table's perspective they 
> cannot talk to all the hosts on that network.  They need to expand their 
> netmask if they want to see the whole network.
>
> > i.e. it's all an implementation detail for the most part and there's a 
> > lot of allowed flexibility here.
>
> You might get a wee bit further with this line of thinking if you were 
> to change the question slightly and  suggest that the gateway address is 
> _not_ on a different subnet but rather is on the subnet with the larger 
> netmask.  Then essentially you don't have two networks any more, just 
> one.  The smaller subnet is simply a logical creation invented elsewhere 
> in the network, and it is irrelevant to the host in question.  So long 
> as the ethernet and ARP will allow the host in question to see every 
> host on the whole local area network (that it needs to see), then you're 
> done.
>
> I.e. just pick the largest netmask, the one that allows you to "see" the 
> IP address of the default gateway as if it is on the same network as the 
> host, and be done with it!
>
> So, if I'm seeing this right then all I can say is that this artificial 
> subnetting of a larger IP network which is all actually on the same LAN 
> into smaller subnets is just plain unnecessary and should even be 
> described with many other nasty words that I won't use right now.

Well as i read the setup example in the guide, thats why we specify a 
route to a host and not a network. first on the network not "attached" 
to an outside network, uses this network settings to reach a "gateway"
Dont know if it is a proper gateway follwoing exact definitions.

Then it asks to be directed to that networks gateway _host_, with that 
networks, settings. So the network mask of  the different networks 
wouldnt matter,
And the request is then led out to to a outside network.

Isnt that so?

> > 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.
>
>
> A proper cloud of networks all connected to the same interface will 
> require separate host addresses for each network; at which point 
> specifying different gateways (for specific networks) on each of those 
> networks is already supported.  You still only get one default route, of 
> course, but that's sensible too.