August 24, 2008

Yes, You Can Use Two Routers With DHCP and Without Bridge Mode.

Filed under: Sci/Tech — PolitiCalypso @ 4:10 pm

Do a web search for “how to use two routers on a home network” or something similar, and the overwhelming majority of the search results will be blog posts or forum topics directing you to put one of the routers in “bridge mode” and/or to turn off DHCP.

(Bridge mode is a mode that, in a nutshell, has the device simply forward whatever traffic it receives from the “downstream” router. It effectively disables that router as a recognizable network device. Rather than a true “stop” on the network, it is a bridge. You can then forget connecting anything to that router and expecting to get a functional Internet connection, unless you have a non-bridged router in between. DHCP is a protocol that automatically assigns IP addresses to machines on a network. The idea behind turning it off for one of the devices is to avoid getting conflicting addresses assigned to machines.)

I have a rather unique situation in my home network, and it is such that neither bridge mode nor deactivating DHCP would work well for me. The network has two devices with routing capability — a Westell 6100 combined DSL modem/network router with a single Ethernet port, provided by the ISP, and a Netgear WGR614 wireless router that I own. I had been using the Westell in bridge mode, with the Netgear acting as the DHCP server and the router for the network. There are four computers that could conceivably connect to the network, two of them requiring wireless because of their locations, and the Netgear’s capabilities were clearly required.

There was a problem, though, and a pretty significant one. During hot periods of the year, the Netgear has a tendency to overheat and lock up every 25 minutes or so. It does it even after I modified the router to install a small CPU fan — IF it has been kept online for hours and hours. Like overnight, when no one is using the Internet. Leaving the thing off for about 12 hours tends to eliminate most overheating problems, but leaving it on nonstop pretty much guarantees them during the right time of year.

Although there are four computers that could use the Internet at any time, very rarely are all four of them actually doing so. There is a large stretch of time during the day when only one computer is online, in fact — a computer that is close enough to the router and modem to have a wired connection rather than wireless. My line of thinking was, “If the Netgear overheats when it’s been left on too long, and there is only one computer that is online during large stretches of time, why have the Netgear on all the time? Why not connect that computer to the Westell modem+router and turn off the Netgear?”

Ah, except that the modem is in bridge mode in order to work with the wireless router. Sure, it can be switched out of bridge mode if the computer needs a direct connection to it, but if the Netgear is to be used later, it must be put back in bridge mode. Same for disabling DHCP on it. Every time the network configuration was changed, the settings for the modem would have to be changed. What a royal pain. There must be a way around this, right?

Well, yes. There most definitely is.
The first step is to stop thinking of the Westell as a “modem.” It is, but the functionality of interest for me was its routing capability. Even though it has only one Ethernet port and one USB port (and could not use them both at the same time), it is still a full-featured (in terms of software) router. It could interface with other routers, configured properly. It could exchange information with them, learning the current network configuration through this exchange, whatever that configuration might be. Downstream Netgear router with four computers connected or direct connection to a single computer, the Westell could be configured to learn what the network currently looked like, and it would not have to be reconfigured and rebooted every time something changed.

OK, OK, Googlers, you want to know how to do it, right? Here you go. Keep in mind that this is for a changing network topology with two routers, and one of those routers doubles as your DSL or cable modem, though it should work whether that last is the case or not.

For reference, here is a picture of the two possible configurations of my network. (The left configuration simply indicates the possible total number of machines that could be online at once.) What I wanted to do was to set up the two routers so that I could switch back and forth between these configurations, in order to take the Netgear out and cool it off when only one machine needs to be online.

1. Keep DHCP enabled on both routers. This is contrary to what most of your web searches will tell you, but the bottom line is that unless you want to assign static IPs to everything on your network, you will need DHCP turned on for both routers. After all, the idea is that there is at least one computer on the network that could be connected to either router.

2. If both devices have DHCP turned on, and they are using the same address family, then there will be conflicts even if you tell them to use different address ranges for the last octet (the last number in an IP address). To avoid this, either implement subnetting, with correct subnet masks, or assign different address families to the two routers. I am not currently bothering with subnetting, so I have assigned the Westell to use 192.168.1.254 (the default IP address) as its own address and assign addresses to downstream devices from a DHCP pool of 192.168.1.1 – 192.168.1.253. The Netgear, on the other hand, is assigned 10.0.0.1 and assigns addresses from 10.0.0.2 – 10.0.0.50 via DHCP. Please take note that these settings are done in the respective configuration pages for each device; i.e., I do not want to set the Netgear’s address via the Westell config page. In any case, when the Netgear is connected, the Westell will assign an address to it from its own pool of 192.168.1.1-253. In effect, the second router will have two IP addresses, one that it assigned itself, and one that the upstream router uses to recognize it.

3. Turn on RIP, the Routing Information Protocol, on both routers. Make sure that they both are set to transmit and receive, and that the version of RIP is the same for each. This allows them to exchange their routing tables and learn about what the network currently looks like.

4. If both upstream and downstream routers have a setting to “log on to the Internet” using your ISP username and password, activate it only for the one that will always be part of the network, which ought to be the upstream router. If your situation is like mine, with the upstream router being a DSL or cable modem as well, then this is the one that you want to have log on to the Internet. The Netgear router had this setting, but because the Westell is always part of the network and the Netgear is not, I didn’t want it turned on for the Netgear.

One nice side benefit of doing all this was that it allowed me to access the configuration pages for the Netgear and the Westell (if the Netgear was on the network). With the Westell in bridge mode, I was unable to get to its configuration page from behind the Netgear, and I would have to take the Netgear out of the network and connect directly to the Westell to do so. The Westell page told me a great deal of information about the DSL connection, since the device doubled as a modem, and it was a minor annoyance to not be able to look at that from behind the wireless router.

In any case, that’s it. That’s what you do to have a smart network that changes configuration and uses more than one router some of the time. If this is what you need, like it was for me, then don’t be fooled by the forum posts and blogs telling you to turn off DHCP or put one of them in bridge mode. That only works if your network does not significantly change its topology, and it will NOT work if you need a computer connected to that router. Do this instead. It works, I promise.

12 Comments »

  1. nice summary. I would like to do this for no other reason than the ‘side benefit’ you mention. Ever since Verizon started installing FIOS in my area, my dsl service has periodically been dropping out, for a couple days at a time. To make sure it’s not a router issue, I go through the process a connecting directly to the modem, switching modes, etc., just to verify that my router’s not the problem. This will make some of that go a lot smoother.

    Comment by NJank — September 2, 2008 @ 6:20 am

  2. My modem’s config page could be accessed by typing “http://launchmodem” (no .com), but I had to add an entry to my HOSTS file in order to use that when I was behind the router. Using the modem’s IP address always works, though, of course. Thanks for commenting and good luck.

    Comment by Erin (PolitiCalypso) — September 3, 2008 @ 9:28 pm

  3. This is a very stupid way to do it either way. This is how it should be done: 1. Leave the routermodem with its default settings. 2. Configure the Netgear as a switch with an accesspoint, this is done very easy by disabling the dhcp-server and give it a static ip in the same range as your routermodem e.g 192.168.1.253. Then don’t use the wan port when you connect the routermodemo the Netgear, it’s very important to now use one of the 4 LANports. This configuration works on any WLAN router. Second, if your Netagear is not stable, you should throw it a way and buy a new one.

    Comment by Øyvind — November 6, 2008 @ 9:10 am

  4. You’re certainly a polite one… do you go from blog to blog calling the author stupid?

    In any case, your advice does not resolve the problem, which was to have both routers acting AS ROUTERS. I am a software engineer and I assure you, if I had wanted it to act as a switch, I would have set it to act as a switch. As it happens, I don’t. There are reasons for why it was necessary to have it act with its full functionality.

    And I don’t know exactly where you are located, but in my corner of the world, we’re in a recession. I have blogged extensively about it here. We don’t throw things away unless we have no other choice, because there are limited funds. You come off as exceedingly callous to that fact.

    Comment by PolitiCalypso — November 6, 2008 @ 11:49 pm

  5. Erin,

    Thank you very much for this post. I have been frustrated trying to figure out how to setup two routers with dhcp enabled. I am sharing a townhouse with 3 other people, and there is one router for all of us to access. I wanted to setup a router for all of my computers and devices so I could easily share printers and files between them without having to setup a bunch of complicated permissions, or worry about the other people getting access. This blog post helped me to do that.

    Thanks,
    A Fan of Greek Mythology

    Comment by Odysseus — August 21, 2009 @ 8:02 pm

  6. Thanks This works but may i ask is this safe on both 1st and 2nd router?

    Comment by SMB — November 17, 2009 @ 11:40 am

  7. The post is now out-of-date for my personal setup, as the netgear in question got replaced by a faster router. However, when I was doing this, I didn’t find any security or hardware safety issues. It will increase the traffic load, so if overheating is an issue, you might take that into account.

    Comment by PolitiCalypso — November 17, 2009 @ 9:58 pm

  8. Thank you for this. I have been struggling to accomplish this for some time. I will try at home because the adsl ruter/modem has only 4 ip addresses in its DHCP pool. Also, I have a wireless router with a more decent DHCP ip address pool.

    In relation to cabling:

    Should I connect in the WAN port of the wireless?

    I read in another thread that if I connect to a numbered port, basically I am getting rid of the routing capabilities.

    Comment by Miguel Alcaine — April 12, 2010 @ 3:24 am

  9. Hi, just wanted to tell you, I liked this article. It was inspiring. Keep on posting!

    Comment by Belkin Router Ip — January 14, 2012 @ 8:45 am

  10. Hi

    I´m trying to connect two routers WIRELESSY, i.e. not using a cable between the routers. Reason for this is that Router #1 (also including ADSL modem) is in the kitchen and Router #2 is 15 meters away in the study together with the “main computer” that lacks a wireless network adapter. So in reality I´m looking for a way to use router #2 instead of buying a new wireless network adater for the main computer.

    Does anyone know if the DHCP configurations described above will solve this problem for me? Or any other way?

    Router #1 is a brand new ZyXEL P-2601HN-F1 that my ISP provided, router #2 is an older one, TP-LINK TL-WR64xG(x=1 or 2).

    I´m not an expert in any way regarding computers so any help will be deeply appriciated. Thanks.

    Comment by Re-user — April 25, 2013 @ 4:39 am

  11. This is an interesting read but I am also curious as to why Øyvind’s suggestion wouldn’t work (well not counting the complete lack of any kind of tact). It would seem as if the Westell was set as a DHCP and the Netgear as a “switch” then it would end up doing the same thing as far as being able to server a single wired client.

    Comment by Slava — May 10, 2013 @ 10:56 am

  12. How do I find the assigned IP of the secondary router. I use 2 d link

    Comment by LOLmaster — December 6, 2013 @ 11:39 am

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