Re: Multi-Networking in Linux - Multiple Subnets

On 2026-07-31, c186282 <[email protected]> wrote:
> On 7/30/26 17:18, Lawrence D’Oliveiro wrote:
>> On Thu, 30 Jul 2026 13:58:28 +0200, Carlos E. R. wrote:
>> 
>>> The ISP configures the router so that it is mandatory to use
>>> 192.168.1.*, and they take a good chunk of it for their own usage. I
>>> have no recourse but using this non standard network.
>> 
>> I remember seeing one setup -- I think in the house of a friend of a
>> friend -- where there, not one, but *two* routers between the house
>> LAN and the Internet. The outer one specified one private LAN, while
>> the inner one put the house machines on an entirely different one.
>
>    TWO routers ? UUUGGGH !!! Sounds like
>    a world of trouble !

Sounds can be deceiving.  If the routers are arranged in series,
you get what is sometimes referred to as double-NAT.  You end up
with three levels of network:

- outer WAN with IP address provided by the ISP

- middle ground, LAN to the outer router and WAN to the inner
  router

- inner LAN

A host in the middle ground can access the rest of the middle
ground and the outer Internet.  The middle ground can be used for
less-trustworthy machines, like an employer's laptop or other
Windows machines.  It's somewhat like a DMZ.  If printers must be
accessed by the employer's laptop or other Windows machines, the
printer can go in the middle ground.

A host on the inner LAN can access the rest of the inner LAN, the
middle ground, and the outer Internet.  For an intruder to attack
a host on the inner LAN, the intruder needs to break through both
routers.

Zoom and a few other services don't play well with double NAT, so
Zoom and the like bad-mouth double NAT.  If I ever really need to
use Zoom, I'll use it from a Raspberry Pi in the middle ground.

-- 
Robert Riches
[email protected]
(Yes, that is one of my email addresses.)