How to setup a two router network?

I was trying to setup a network for a friend recently.. His ISP gave him a DSL modem/wired router combo, but he wanted to have a couple wireless computers on the network, so he got a linksys wireless router.

I plugged the uplink of the linksys into one of the ports on the DSL modem/router and then plugged a nearby computer into the linksys. I could see the local network behind the linksys just fine, but could not get out to the internet.

I couldnt connect to the DSL modem/router's configuration page while behind the linksys, so I plugged the computer into another port on the DSL modem to get access and see what I could change to fix the problem.

On the DSL configuration page, it showed that it had found the router and assigned it an IP address. I noticed some options to setup a default DMZ server, cause the modem to act as a bridge, etc.. I tried messing around with these (made the router the default DMZ server, bridged the connection, but nothing seemed to work.

I plugged the computer back into the linksys router and brought its configuration page up. There was a page with an option to make the router a "router" or a "gateway". It was set on "gateway", I tried switching it to "router" and enabling RIP. This did not seem to make much of a difference. It let me bring up the routing table. It had a "source gateway" and "destination network" field. The destination network field had 192.168.1.0... The other field I believe had the internet address of the DSL modem.. not sure.. I had the option to manually configure the table, but was not sure what to do at this point.

Im fairly good with both hardware and software on computers (Im a hardware technician, and I do web and database design on the side), but when it comes to networking I am still pretty clueless... Can any networking gurus help me out here? Im really interested in learning.

 

Simply put, if you have 2 routers connected to eachother, you have to put them on separate network subnets.

Here's what you do.
The modem/router from the ISP will have an external connection (internet) and then an internal one (LAN). THe internet side gets an IP assigned to it from your ISP. This can be anything, like 69.135.54.232. The modem/router assigns the LAN side will likely use IPs in the 192.168.1.x range. Any device (computer or router) hooked up to it will get a 192.168.1.x address.

Now the linksys works much the same way. By default, the LAN side will try to use 192.168.1.x address. But the problem is that the internet side of the linksys will be getting a 192.168.1.x address from the dsl modem. both sides will be on the same network, and the router won't know what to do. (it creates a routing loop)

The solution is to change the IP addresses that the linksys gives out on its LAN side. You should be able to do this in the configuration. Somewhere is an option for DHCP settings. Change the IP range to 192.168. 2.x with a subnet mask of 255.255.255.0.
This will make it a separate network from the IP that it is getting from your modem. And the linksys won't get confused.

 

Oops... thats one thing I forgot to mention.. I had thought about that and changed the subnet mask for the linksys router:

DSL router was: 255.255.0.0

Linksys was: 255.255.255.0

So the linksys was assigned a completely different ip address...

But did I screw the thing up by having two different subnet masks on the local network?

 

let the DSL modem handle DHCP, so just leave it to default settings.

step 1: log into linksys box and give it a new IP, I usually use one after the other router(the modem) so if the modem defaults to 192.168.1.1, then give the linksys 192.168.1.2 so you know how to get back to it. After you do that you'll have to log back into it on its new IP.

step 2: disable the DHCP

step 3: change the mode from "gateway" to "router" (very poorly worded by linksys IMHO but its what you need to do, should be labeled router/switch or something of that effect).

step 4: plug the cable from a LAN port on the modem to a LAN port on the linksys, NOT the WAN.

That turns the linskys into an access point essentially leaving the modem to handle all the routing and IPs, done it countless times in many installs.

 

Thanks ZJ, that was a big help.. My problem was I used the WAN port..

Unfortunately I won't have access to the setup again anytime soon to test it, but I got a good feeling that would work

 

I am a system integrator and run all the home networking jobs at my company, only use linksys so that why I told it to you that way, its the way I've done it countless times. In general I'd recomend turning everything off in the modem and letting the router handle the routing and firewall but I don't know if they needed the ports on the modem or anything.

 

My way will work too...
But as i re-read what i typed, i can see it was a bit confusing.

 

Just out of curiousity though.. Would it be possible to actually stack two routers in 'gateway' mode? If so, what would you need to do? Routing table?