I recently moved from DSL to Cable. I have a 'dumb' cable modem, but a 'smart' DSL modem (two actually). What I would like to do is to connect the cable modem to one of the lan ports of the DSL modem (the internet port of the DSL modem is a DSL line obviously) and have it act as the DHCP and wireless router.

The modem in question is an ActionTec GT784WN.

The closes answer I've found seems to be this one, but it seems to be discussing using the modem/router as a wireless access point instead of as a router.

  • What distinction are you referring to when you describe a modem as "dumb" versus "smart"? – sawdust Dec 6 '14 at 21:38
  • Not 100% sure. It seemed to have been used in a couple of posts (such as the one linked). I think it basically means router-less. – Nate Diamond Dec 6 '14 at 22:17
  • In that link you provided, it uses the phrase "dumb wireless router", which does not fit your interpretation of "dumb", i.e. "router-less wireless router". What is the manufacturer & model of your cable modem? – sawdust Dec 7 '14 at 14:15
  • Motorola SB6121 – Nate Diamond Dec 7 '14 at 21:38
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    No, that will not work, unless you can find capability to turn a LAN port into a WAN port as with this DLink modem. There are kludges such as using a PC as in superuser.com/questions/365046/…. But that will not provide a proper firewall with DHCP and NAT of a real router. I.E. test your network vulnerabilities with Shields UP! – sawdust Dec 8 '14 at 0:21

You might be able to do it, but even if you do, it might not work very well. To understand why, you'll need to understand a little about routing.

The purpose of a router is to move packets along the proper links, to get them closer to their destinations. But there are different types of routers that play different roles. There are the backbone routers, for instance, which form the core of the Internet; they take packets from many different places and send them to many different places, and for this purpose they maintain huge tables representing the structure of the entire (worldwide) backbone.

Then there are gateway routers like your DSL modem, which only need to know about the one network they sit on the boundary of. For any data going outside that network, your router will have a "default route", which effectively says, "If the address on this packet doesn't match any of the addresses for which I already have a more specific route [i.e. the addresses in this network], send it upstream," where "upstream" on a DSL modem usually means out to the DSL link and on toward your ISP. Unlike the aforementioned backbone routers, a DSL modem has a built-in notion of "upstream", and it's usually coded to always be on the DSL link.

Now, with some modems, it's possible to alter the routing table by hand so the default route points to an Ethernet port that you would set aside for the role. I know I could do that with mine, for instance, because it has a Linux-based firmware with a command-line shell option; so I could log in and use shell commands to change the routing table. But this change wouldn't survive a reboot, or a configuration change of any kind, for that matter; it's very much a "hack". Even if your modem allows this as well, I'd be surprised if it worked any differently in this respect.

A possible permanent solution would be to install a custom firmware that does what you want. The most obvious problem there is whether one even exists, and if it does, where you can find it. Then, of course, you have to trust it not to have some sort of malware. And there's the risk of bricking the modem if you make some sort of mistake, although it might have a recovery mode.

The easiest, and most likely best, solution is to get a standalone router. It may have desirable features you wouldn't have in the modem, and more importantly, it may have better security (assuming you choose correctly).

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  • That's interesting. I think I can actually modify the routing table directly from the router gui. I'll give that a try and see if it will work. – Nate Diamond Dec 7 '14 at 21:38
  • Like I said, modifying the routing table is a hack. Also, someone brought up a very important point: it won't be properly firewalled (its filtering rules make the LAN ports "trusted", since they're supposed to be inside the network). I would not try it seriously. Better get a proper router. – Daniel Dawson Dec 8 '14 at 18:44
  • Yeah, that's what I think I'm going to do. We can live with no wireless for a bit. – Nate Diamond Dec 8 '14 at 18:50

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