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Re: Quick question about routing

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On 2019-04-27, szr <szr@sREzMOrVEoman.com> wrote:
> Ok, I have a LAN with the following notes:
>
> 192.168.1.254 - Main router / default gateway for LAN
> 192.168.1.4   - Linux server (eth0)
> 192.168.1.8   - Windows laptop
>
> Linux server (192.168.1.4) has ip(v4) forwarding enabled.
>
> As an experiment I added an ip, 10.0.0.4, as eth0:0.
>
> Then I added a static route for 10.0.0.0/8 using 192.168.1.4 as the 
> gateway on the main router.
>
> After that, I ran `ping 10.0.0.4` from the Windows laptop and the ping 
> was successful.
>
> While I do have plenty of networking experience and I'm not surprised 
> that this worked, one thing sort of bugs me.
>
> I'm able to ping an ip in the 10.0.0.0/8 subnet from a host that only 
> has an ip in the 192.168.1.0/24 subnet. I know that the laptop is using 
> the route from the default gateway (the main router) but how exactly is 
> the ping getting back when the source of the ping isn't coming from 
> another 10.0.0.0/8 address? Is this more due to ip forwarding, or 
> because it's on the same physical network segment?
>
> Again, I otherwise have a very good understanding of networking, but I 
> can't seem to resolve this scenario completely in my mind, and would 
> really like to know what exactly is happening here that allows the round 
> trip to occur successfully.
>

Do
route -n
on the various machines. I suspect that you will have something like 
192.168.1.0     0.0.0.0 ...
10.8.0.0        0.0.0.0 ...
These say--If you get and address which, when anded with the mask give
thos addresses, just send the packet onto the network connected to the
machine-- well actually, it sends out a request onto the local network
sayins "Who has 10.8.0.5" and the machine that has that address sends
back and says "I, who has MAC address 23:55:12:ac:0d, have that address"
and your machine sends out a packet addressed to that MAC address.

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Thread

Quick question about routing szr <szr@sREzMOrVEoman.com> - 2019-04-26 19:17 -0700
  Re: Quick question about routing William Unruh <unruh@invalid.ca> - 2019-04-27 03:02 +0000
  Re: Quick question about routing Pascal Hambourg <pascal@plouf.fr.eu.org> - 2019-04-27 09:14 +0200
    Re: Quick question about routing Theodore Heise <theise@panix.com> - 2019-04-27 14:28 +0000
    Re: Quick question about routing szr <szr@sREzMOrVEoman.com> - 2019-04-27 08:03 -0700

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