DSL modem -> wireless router network problems

[Grant W. Petty]

This question is similar, but not identical to some other messages I've
seen here. I hope someone can help. I spent two hours in chat support
with Linksys only to be abruptly disconnected without a solution to my
problem.

I have a 2wire DSL modem that is hardwired (ethernet) to the WAN port on a
Linksys WRT54G wireless router. The router in turn talks to (a) a desktop
via an ethernet cable and (b) a laptop via the wireless broadcast.

Both computers can reach the internet with the current configuration. I
only ran into trouble when I tried to take the next step of getting them to
talk to each other. Unfortunately, they can't seem to see each other at
all (even though they're both nominally in the same workgroup), so file
sharing etc. is out of the question until I get this fixed.

The DSL modem "sees" both computers, and from its perspective, both have
sequential IP addresses of 172.16.1.33 and 172.16.1.34.

What I discovered after several days of groping, and what *may* be relevant
(not sure), is that the desktop (hardwired) PC apparently gets *its* DHCP
IP (as reported by ipconfig) from the router, while the wireless laptop
apparently gets it from the DSL modem (but through the router, obviously).

That is, ipconfig on the the laptop yields an IP consistent with the second
of the above addresses, while ipconfig on the desktop shows 192.168.1.100,
which turns out to be the default IP provided by the router's internal DHCP
server. And apparently the router only does this for the wired
connections, for some reason.

I thought, if I disable the router's DHCP on the wired connections, maybe
the desktop will pick up the DSL modem's assigned IP just like the laptop
already does. But this doesn't seem to have worked, as ipconfig /renew
failed with a timeout and I had to hard-reset the router to reestablish a
connection.

What I'm not clear on is whether there's supposed to be a fundamental
difference between the way wired and wireless clients on the same router
get their IP addresses and whether any such difference is responsible for
the problem I'm having. Is the router is supposed to translate between the
incompatible IP addresses for the desktop transparently, or do I need to
find a way to force the router to pass the DSL modem's assignment on to the
desktop?

I'm a network newbie, so any insight will be much appreciated.

- Grant

 

[Kurt]

Something is connected wrong if your description is accurate. DHCP is a
broadcast-based protocol and only works on non-routed network segments
(unless something is configured specifically as a DHCP relay). So if both
computers are on the LAN side of the router, they should only be getting IP
addresses from the router. The DSL Modem shouldn't be able to "see" either
computer, it should only see the router. The router (if the WAN interface is
configured for DHCP) should get it's WAN address from the modem. And if
they're both handing out private addresses, you probably don't even need the
router, just a switch.

....kurt

 

[Grant W. Petty]

Something is connected wrong if your description is accurate. DHCP is a
broadcast-based protocol and only works on non-routed network segments
(unless something is configured specifically as a DHCP relay). So if both
computers are on the LAN side of the router, they should only be getting IP
addresses from the router.

Hard me for to say, since I knew nothing about the subject before starting
in on this. All I can say with certainty is that if I configure the DSL
modem to use a particular range of private IP addresses, that configuration
is reflected in the IP the laptop receives (as reported both by ipconfig
AND the DSL modem's interface, which shows all devices on the network).
But what the desktop gets (as reported by ipconfig) is impervious to the
settings on the DSL modem and is instead controlled by the Linksys WRT54G.
Note incidentally, that the four ethernet ports on the latter device have
been described to me by some as a "switch" not a router (not that I fully
understand the difference).

The DSL Modem shouldn't be able to "see" either
computer, it should only see the router.

Again, all I can say is what I see. There's a URL that I can point to with
my browser that accesses the DSL modem GUI (the URL is gateway.2wire.net).
From that GUI, I can change the DHCP settings of the 2wire DSL modem and
also display the characteristics of all devices on the LAN, including the
IPs that the modem 'thinks' they have and whether they're active or
inactive. As noted before, what the modem thinks the wireless laptop has
agrees with what ipconfig reports. The same is not true for the desktop
that is wired to the router/switch.

The router (if the WAN interface is
configured for DHCP) should get it's WAN address from the modem. And if
they're both handing out private addresses, you probably don't even need the
router, just a switch.

You could be right. I have to assume that Linksys knows what it intended
with its WRT54G. Whether that intent is compatible with the intent of the
2Wire DSL modem is a question I can't answer. That's why I'm looking to
the experts for help (note that even the Linksys support people seemed
stumped by my question, like they had never seen this before).

- Grant

...kurt

________________________________________
Prof. Grant W. Petty
Atmospheric and Oceanic Sciences
1225 W. Dayton Street
University of Wisconsin-Madison
Madison, WI 53706

(e-mail address removed)
Tel: (608) 263-3265
Fax: (608) 262-0166

 

[CWatters]

Most home LANs are set up as follows...

The router gets an IP address for it's WAN port from the modem/ISP.

On the LAN side of the router your computers get their IP address from the
Router (which has DHCP enabled). These local addresses are normally of the
form 192.168.0.x

The router performs some magic called Network Address Translation (NAT) to
make sure packets sent to it's WAN IP from the web are routed to the right
PC on the local LAN.

In the absence of a dedicated server, computers on the local LAN are
configured to be part of a Workgroup (no spaces in the computer names or
workgroup names).

** To allow file sharing you may need to set up identical user accounts on
all PC ** My system is set up as above and it was this last bit that
stopped me sharing files.

NAT works fine for most people but it can cause problems with some video
conferencing applications. I think this is when having one WAN IP for each
computer might be helpful - I've no experience with setting that up but I
assume you would have to turn off NAT and DHCP in the router somehow. I
would avoid doing this unless it's really really necessary.

 

[CWatters]

(note that even the Linksys support people seemed
stumped by my question, like they had never seen this before).

I might be wrong but I think it's quite unusual for an ordinary DSL modem to
act as DHCP server and be handing out IP addresses. Are you sure it isn't a
router with a DSL modem installed? How many ports are there on it?

 

[Guest]

It is true that the router can allow the dsl modem (acting as a proxy dhcp
server) to assign the dhcp addresses instead of assign it by itself (not
usual and not the default configuration for linksys) in that case the router
would need to be configure to be doing transparent bridging or to do dhcp
relay. As per your description this is not your case because the router is
assigning the ip for the desktop. If you do an ipconfig /all from command
prompt (DOS) in the laptop what's the ip address for the dhcp server? is it
the router or something else? whether if the dsl modem sees both PCs or not
is not really important since the router may be doing static translations
(again not the default configuration). For the computers to be able to
communicate between them they must be able to ping each other first, if you
ping from station a to b what are the results? the router should allow
netbios and netbui and the computers should be configured to run it. When
all those issues and configurations are done then you set up the same
workgroup and proper users and pw. Let us know the results of the ipconfig
/all and pings test as well as if there are any restrictions from the router
config to pass traffic to the stations. Cheers!

This question is similar, but not identical to some other messages
I've seen here. I hope someone can help. I spent two hours in chat
support with Linksys only to be abruptly disconnected without a
solution to my problem.

I have a 2wire DSL modem that is hardwired (ethernet) to the WAN port
on a Linksys WRT54G wireless router. The router in turn talks to (a)
a desktop via an ethernet cable and (b) a laptop via the wireless
broadcast.
Both computers can reach the internet with the current configuration.
I only ran into trouble when I tried to take the next step of getting
them to talk to each other. Unfortunately, they can't seem to see
each other at all (even though they're both nominally in the same
workgroup), so file sharing etc. is out of the question until I get
this fixed.
The DSL modem "sees" both computers, and from its perspective, both
have sequential IP addresses of 172.16.1.33 and 172.16.1.34.

What I discovered after several days of groping, and what *may* be
relevant (not sure), is that the desktop (hardwired) PC apparently
gets *its* DHCP IP (as reported by ipconfig) from the router, while
the wireless laptop apparently gets it from the DSL modem (but
through the router, obviously).
That is, ipconfig on the the laptop yields an IP consistent with the
second of the above addresses, while ipconfig on the desktop shows
192.168.1.100, which turns out to be the default IP provided by the
router's internal DHCP server. And apparently the router only does
this for the wired connections, for some reason.

I thought, if I disable the router's DHCP on the wired connections,
maybe the desktop will pick up the DSL modem's assigned IP just like
the laptop already does. But this doesn't seem to have worked, as
ipconfig /renew failed with a timeout and I had to hard-reset the
router to reestablish a connection.

What I'm not clear on is whether there's supposed to be a fundamental
difference between the way wired and wireless clients on the same
router get their IP addresses and whether any such difference is
responsible for the problem I'm having. Is the router is supposed to
translate between the incompatible IP addresses for the desktop
transparently, or do I need to find a way to force the router to pass
the DSL modem's assignment on to the desktop?

I'm a network newbie, so any insight will be much appreciated.

- Grant

--


2nd Law of Thermodynamics: Chaos will Reign.

///////////////////
--Anthrax--
//////////////////

 

[Kurt]

Everything CWatters said, plus if the DSL Modem is also a router, it is
redundant to have a second router. Make sure you are connected as such.

1) Broadband router WAN port connected to DSL LAN port.
2) Wired computer connected to broadand router LAN port
3) Wireless computer connected wirelessely to broadband router.

Enable DHCP on both (they must be on different IP networks)

Boot up and do an ipconfig /all from both. Let us know what you get.

If that works, and your modem is also a router, you might be able to use
your wireless router strictly as switch (most are both). This will
eliminate "double NAT" (double NAT will work for simple Internet
connectivity, but complicates things if you need inbound services like a web
server, VPN, RDP, etc.).

....kurt

 

[Grant W. Petty]

I might be wrong but I think it's quite unusual for an ordinary DSL modem to
act as DHCP server and be handing out IP addresses. Are you sure it isn't a
router with a DSL modem installed? How many ports are there on it?

After a week of struggling, this question finally gave me the breakthrough
I needed.

I had alway assumed that the HomePortal provided to me by SBC/Yahoo was
"just" a DSL modem. I specifically bought the Linksys WRT54G wireless
router because someone advised me that I needed it in order to be able to
add wireless to my laptop. I have just discovered to my chagrine that the
HomePortal has built-in wireless. All the time that I thought my laptop
was talking to the WRT54G, it was actually talking to the HomePortal. It
explains a lot of mysterious behavior. Needless to say, I feel pretty
stupid.

I've taken the WRT54G completely out of the picture, and my laptop now sees
my desktop, which is what I had been trying to achieve all along.

Thanks to everyone for their insights.

- Grant

 

[CWatters]

I've taken the WRT54G completely out of the picture, and my laptop now sees
my desktop, which is what I had been trying to achieve all along.

Jolly good. Thanks for letting us know. Nothing worse than not knowing how
it went.