A bad idea? Daisy-chaining modem and router with the same ip numbers

[Geoff Shang]

On Sat, 26 Jan 2013, Steve G. wrote:

> if I take a modem that runs the LAN as 192.168.1.1, and plug into one of
> its ports a wireless router ALSO running as 192.168.1.1, would I bring down
> the Internet or cause other types of horrible harm?

The Internet at large wouldn't be affected, but you'd cause yourself 
problems.   Packets from machines connected to the router which 
are destined for the modem would probably never get there.

> The fixed IP desktop is fine, because the new and old networks are the
> same. The wireless devices are fine, because they run on the new network
> and get the IP automatically assigned. But the server is out of each, as
> its address is 192.168.1.190 and it is connected to a router with the
> network 192.168.2.x
>
> So I am thinking what might happen if I change the router's network back to
> 192.168.1.x - would it work?

You could change the router back to 192.168.1.x but the router itself 
cannot be set to an address that any other device has.  You need to avoid 
address collisions.

I guess you could also leave your router address as it is but set its 
netmask to 255.255.252.0 which would then include the 192.168.1.x network 
as well.  This might needlessly complicate things though.

If you can avoid address collisions, setting everything to the same 
network would work fine.

If the modem can do DHCP, you could disable DHCP on the router and let the 
modem take care of handing out addresses.  Then you could just give the 
router an address that won't collide (e.g. 192.168.1.254) and just get it 
to pass IP traffic back and forth.  This is what I did here when I ran out 
of ethernet ports on the modem/router.

HTH,
Geoff.