I have a modem-router device, the device is connected to a DSL phone line to connect its router to the internet, the device in this setting has a public ip address (provided by the ISP) and a local subnet

Now any PC can connect to internet through this device by connecting an Ethernet cable and obtaining a local IP address from its subnet. Simple....

The problem,

I have a SonicWall NSA 220 device that I need to install behind the modem-router device.

I need to have my local network connected to the SonicWall then in turn have the SonicWall connected to the modem-router device to access the internet. How do I configure the SonicWall WAN port to do this?

P.S. I managed to configure a SonicWall interface to LAN zone, and now I have local network with subnet, so the question in other words, how do I get this network connected to the internet?

1 Answer 1


The best answer will be to ask your ISP to either exchange your modem-router combo device for a modem-only device OR see if they can switch it into Bridge mode (where it becomes only a modem and no longer acts as a router). [I have a Cisco modem-router device from my cable ISP that is like that, I called them and they did something on their end then had me reboot it and it was no longer a router]

If you CAN'T do that (or if you need to get up and running fast) then you CAN connect your SonicWALL behind the modem-router device, it will be double-NAT. Some services will be less happy about that but most won't care.

Just configure X1/WAN with an IP on the subnet. Your LAN clients (on will be NAT'd by the SonicWALL to a 10.0.0.x IP and then NAT'd again by the modem-router device to your actual public IP.

For outbound this isn't an issue. For INBOUND you will need to open ports on your modem-router (pointing to your X1/WAN IP) as well as create NAT rules on your SonicWALL. Or you can look for a "DMZ" option in the modem-router which forwards all ports to the specified IP (basically a wide-open ANY/ANY/ANY NAT rule) -- common on consumer grade routers.

Note: Some of these modem-router combo devices (which, to be honest are not great, and not designed for business use) will only perform NAT/pass-traffic for IPs it has assigned by DHCP, in that case you may need to set X1/WAN to DHCP and have it obtain an IP from the modem-router device. If yours is a 2wire brand this may be the case.

I have configured a SonicWALL NSA 240 (the precursor to the 220) behind another router doing NAT and it wasn't a problem, I just had to create rules on both. The only limitation is that the SonicWALL won't allow the same subnet on two interfaces so you can't also use for your LAN.

But if I were you I would contact the ISP and ask to either get them to switch it to Bridge Mode (if possible) or exchange it for an actual DSL modem: so the SonicWALL gets the Public IP on the X1/WAN IP (using Static, PPPoE, DHCP, etc). If not you will be limited by the speed/resources/abilities of the combo device which I suspect are not nearly as robust as the NSA 220.

  • Totally agree, Just stick to the first option, ask your ISP for changing your device.
    – KorXo
    Commented May 29, 2015 at 8:59

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