Two Routers, One NAT

[Nipi]

I would like to share this experience and ask one question about one
home network configuration.
I have two routers at home, one provided by the ISP, not wireless,
which I will call router "A"
and another wireless one i recently got for the laptop, I will call it
router "B"
The router A is connected directly to the ISP and there are two
computers connected to the internet through it. And since I can't
connect the router B to the ISP cable directly (their decision) I first
connected the WAN port of router B to one LAN port of router A, which
allowed my laptop to access the internet but i didn't like it that way
because this had to put the laptop behind two NATs.
So i thought of another configuration, to use the router B like a
"wireless HUB only" by connecting one of its LAN ports (instead of the
WAN port) to the router A, which I figured would make the laptop able
to access the ISP's router A directly.
Here came the problem, I noticed that not only the laptop (which is
connected directly to router B) is taking an IP address from the router
B's DHCP, but also the two computers which are connected to the router
A!, and of course this made them all unable to access the internet,
because router B has nothing on the WAN port.
I could solve this problem with one of two methods, either force all
the computers to take IPs from router A, by specifying fixed IPs within
router A's range and use its IP as default gateway, or by completely
disabling the DHCP service on router B, which is a better idea since i
don't need it if i'm using it only like a HUB.
But my question is, what made all the computers including those
connected directly to router A, to connect and take IPs only from the
router B, and on what does it depend that a computer chooses one router
if more than one are on the same network?
(All the computers are running windows XP)

 

[Steve Winograd [MVP]]

I would like to share this experience and ask one question about one
home network configuration.
I have two routers at home, one provided by the ISP, not wireless,
which I will call router "A"
and another wireless one i recently got for the laptop, I will call it
router "B"
The router A is connected directly to the ISP and there are two
computers connected to the internet through it. And since I can't
connect the router B to the ISP cable directly (their decision) I first
connected the WAN port of router B to one LAN port of router A, which
allowed my laptop to access the internet but i didn't like it that way
because this had to put the laptop behind two NATs.
So i thought of another configuration, to use the router B like a
"wireless HUB only" by connecting one of its LAN ports (instead of the
WAN port) to the router A, which I figured would make the laptop able
to access the ISP's router A directly.
Here came the problem, I noticed that not only the laptop (which is
connected directly to router B) is taking an IP address from the router
B's DHCP, but also the two computers which are connected to the router
A!, and of course this made them all unable to access the internet,
because router B has nothing on the WAN port.
I could solve this problem with one of two methods, either force all
the computers to take IPs from router A, by specifying fixed IPs within
router A's range and use its IP as default gateway, or by completely
disabling the DHCP service on router B, which is a better idea since i
don't need it if i'm using it only like a HUB.
But my question is, what made all the computers including those
connected directly to router A, to connect and take IPs only from the
router B, and on what does it depend that a computer chooses one router
if more than one are on the same network?
(All the computers are running windows XP)

You've correctly diagnosed the problem and its solution in a
complicated setup. I agree that disabling the DHCP service on router
B is the best idea.

With router B connected to a LAN port of router A, both routers, and
their DHCP servers, are on the network. When a computer requests a
DHCP address, both routers' DHCP servers receive the request, and both
of them can reply with a DHCP offer. It's then a timing issue: the
computer accepts the DHCP offer that it receives first. It appears
that router B's DHCP server replied sooner than router A's DHCP
server. The result might be different at another time.
--
Best Wishes,
Steve Winograd, MS-MVP (Windows Networking)

Please post any reply as a follow-up message in the news group
for everyone to see. I'm sorry, but I don't answer questions
addressed directly to me in E-mail or news groups.

Microsoft Most Valuable Professional Program
http://mvp.support.microsoft.com

 

[Nipi]

It appears that router B's DHCP server replied sooner than router A's DHCP
server. The result might be different at another time.

I believe you are so right.
Thanks

 

[John Wunderlich]

But my question is, what made all the computers including those
connected directly to router A, to connect and take IPs only from
the router B, and on what does it depend that a computer chooses
one router if more than one are on the same network?
(All the computers are running windows XP)

When connecting two routers in a LAN-to-LAN port style, there are
three settings which should be taken into consideration.

One, as you've discovered, is the DHCP setting. Only the one
connected to the WAN will know the correct DNS (et al) settings.
Disable DHCP on router B.

The second setting is the IP address of the router(s). Many
manufacturers pick the IP address of 192.168.1.1 as the router's LAN
address. Connecting two of these together will generate contention
for this address. Your Router B's LAN IP address should be changed
to an unused address on Router A's LAN subnet outside the DHCP range.

A third setting is on Linksys Routers (maybe others) have a "Gateway
/ Router" mode choice under Advanced Routing configuration. This
should be changed to "Router" for your "Router B" that does not
connect to the WAN port.

Good Luck,
John

 

[Nipi]

Your Router B's LAN IP address should be changed
to an unused address on Router A's LAN subnet outside the DHCP range.

It shouldn't be the same as router A, i agree, but it has to be on the
same subnet, is that only to be able to open it for configuration?
because i have it completely different and everything is fine except
that i can't access the router B for configuration.

A third setting is on Linksys Routers (maybe others) have a "Gateway
/ Router" mode choice under Advanced Routing configuration. This
should be changed to "Router" for your "Router B" that does not
connect to the WAN port.

Nothing like that in my router. What changes when you put it Gateway or
Router?

And one last question concerning connecting two routers on the LAN
ports, i read somewhere lately (http://www.ezlan.net/router_AP.html)
that connecting two LAN ports of the routers requires a crossover cable
but if one of the ports is WAN then a straight patch is needed, but i
used the same cable in both cases and it worked, it's a straight patch
one.

 

[Nipi]

Ok you already answered all my questions in another place