On Sun, 25 Dec 2011 23:27:26 -0600, Char Jackson
wrote:

If the DSL modem already includes a router, why are people adding a
second router? I know there are some edge cases, but I'm wondering
about the majority.

Well, this is messy, but I think you might appreciate the details.
I'll use the common Speedstream 4200 DSL modem as an example. When
installed in the approved AT&T manner, the ethernet port delivers
192.168.1.64 to the external router. The management IP address of the
DSL modem is 192.168.1.1.

When connected to a typical Linksys router, the router also wants to
use 192.168.1.1 as it's IP address. That's not going to work, and the
DSL modem automagically switches to 192.168.0.1 and delivers
192.168.0.64. (This is not 100% reliable, causes some odd problems,
and is largely responsible for why Belkin and others are delivering
routers using 192.168.2.1).

At first glance, this arrangement looks like double NAT. It is, but
with a difference. All IP ports in the DSL modem are forwarded to the
ethernet port, so there's no problem with incoming traffic not making
it to the router. Were this a "real" double NAT setup, the first
router (in the DSL modem) would NOT have any ports forwarded by
default.

The catch is that you can only forward ALL the IP ports to one IP
address. That means that the DSL modem can only do the NAT thing to
one IP address, and therefore to only one device. If that device is a
router, there's no problem. If you try to connect an ethernet switch
to the DSL modem, and plug in multiple computahs, only one computah
will work.

There's one other item that might be of interest. The DSL modem
intercepts all traffic on the WAN (DSL) side destined to the
management IP address (192.168.1.1). Normally, the external router is
configured to send everything to the internet, except the IP's on the
LAN side (192.168.1.xxx). If you plug 192.168.1.1 into the web
browser, the router will send it to the internet, and the DSL modem
will not respond. So, they violate some RFC, and trap this address,
sending it to the local LAN side, and then to the management web
server inside the modem.

The problem is that the 4200 seems to have a botched implementation of
this undocumented feature. The later DSL modems work well, as do most
cable modems. Older modems lack this feature and require a static
route on the WAN side to get to the DSL modem management web page.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558