[CentOS] Simple routing question

Les Mikesell lesmikesell at gmail.com
Tue Sep 4 17:27:20 EDT 2012


On Tue, Sep 4, 2012 at 4:00 PM, James B. Byrne <byrnejb at harte-lyne.ca> wrote:
>
>> That should happen directly without C's involvement if the netmask is
>> 255.255.0.0 on A and B's eth1 interfaces.
>
> It is not.  The netmask on those interfaces is 255.255.255.0.

Netmasks apply to (and describe) connected subnets, not individual
interfaces.   Linux will sort-of sometimes work with mismatched subnet
masks but some things won't see arp broadcasts with the wrong
broadcast address (which again is for the whole subnet).

>>
>>> Instead it goes to Eth0 on C where it dies as one would
>>> expect.
>>
>> Why does C have both internet and LAN addresses on the same
>> interfaces?
>>
>
> I am experimenting to see if this arrangement is workable.  I want to
> know if it is possible to have two separate 192.168.x subnets on the
> same network.

Some things might work sometimes.  You can overlay separate subnets on
the same wire, each with a correct subnet mask, and a designated
router between them, but random things will happen with mixed
netmasks.

> Why?  I do not have a purpose in mind.  I am just
> checking out whether it can work or not.

You would probably be better off using VLANs than overlays in any case.

> If it is impossible then then I will discover why that is so, which I
> think will be useful in itself.

The broadcast address for a subnet is tied to the bits in the subnet
mask, and ethernets need arp broadcasts to work.

--
   Les Mikesell
      lesmikesell at gmail.com


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