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How to connect and have two separate networks with 1 modem and 2 routers?


Aug 29, 2013
How to connect and have two separate networks with 1 modem and 2 routers?
Internet Modem= ARRIS / Motorola SB6121 SURFboard DOCSIS 3.0 Cable Modem
Router 1 = ASUS ASUS RT-AC66U Dual-Band Wireless-AC1750 Gigabit Router (Merlin build)
Router 2 = TP-LINK TL-WDR4300 Wireless N750 Dual Band Router

I own the Cable modem and Router 1. My brother-in-law owns Router 2. He moved in with us ( times are tough). He would like to keep his network ID and network configuration. I have my network set up and don't intend to change it. Can we both have two separate networks using the same cable modem with the following configuration as follows?

ISP to Cable modem ---- Cable Modem to WAN port on Router 1 ---- Router 1's LAN port 4 to Router 2's WAN port----Router 1's LAN ports and WIFI to my computer, PS3, laptop, ect... ----Router 2's LAN ports and WIFI to brother in law's computer ,PS3, ect.

If not what can be done?

Would putting a network switch between the cable modem and the two routers work?

Example: ISP to Cable modem ---- Cable Modem to Network switch ---- Network switch to WAN port on Router 1 and Router 2's WAN port.

Also is it possible to for him to see my shared USB hard drive on two separate networks, or would I have to set up a FTP server for him to see my files. Honestly I wouldn't mind setting up a Secured FTP (SFTP) with my USB hard drive, so I can access my music and movies anywhere.

I would call myself a novice at networking but can figure stuff out quickly. Any help/advice would be nice. Thanks in advance.


Modem to WAN port of your router #1. LAN port whatever, lets say #1 of router #1 to LAN port of router #2. You don't use the WAN port unless it's a DSL/Cable/whatever coming in. Never to connect two routers.

Now comes the tricky part on setting up the second router, because it's going to more than likely confilct with the first.

Each router does DHCP, which means assigning IP addresses, your or whatever. Now if router #1 assigns from say and the second router assigns, the 3rd number being different, then you wouldn't have to switch anything but there would no sharing of anything, no seeing each others PC's on a network, etc.

Ideally, you want the first router to handle the internet connection and the DHCP and let the 2nd router act as a switch with wifi basically.

So you'll have to connect to your brothers router via a cable, give it a static IP and turn off DHCP.

For a static IP for it, try to stick with a number 1 up from what the router/gateway is.

IE. Your router/gateway is, then set the second router to You don't want to set it in the range that DHCP will hand out IP's, usually starting at the 100-255 range, as it may conflict.

It's fairly easy if you can make sense of what I said. lol. I have my cable modem into a router, then 4 more routers in my house connected through, cables, switches, etc to get more WIFI in my huge, old, thick walled house. It's easier just to have a router basically on each floor with WIFI and people can choose what to connect. All the routers are set to static IP, DHCP off, and the main router does all the DHCP/IP/internet work so that all 10 PC's, 9 phones, 4 tablets and 7 XBoxes in my house are on the same network, can all access the file server full of movies and music, etc.


Feb 26, 2014
I am not sure about your particular ISP but most ISP's give out at least 2 IP addresses.

In that case wouldn't it make more sense to put a small switch directly after the modem and then connect each of the routers to a port on that so that they can both maintain their direct access and be accessible easier and independently if one of them wanted to share something like his music for instance.

If this doesn't make sense I would like to know why it would not work as I am planning to implement something like this myself. The big difference for me is that the WAN ip addresses will be fixed which should make it even easier.



Jul 7, 2014
Two Separate Networks with one Modem.

I had a Motorola Cable Modem SB5101 and I have a Netgear WNDR3700 with the Parental Controls activated and I have a Netgear WNR1000 that I wanted for a second network.

I read somewhere that ISP's can give you 2 separate IP addresses. So I called my cable company and asked them about the option for 2 separate IP addresses (one for work purposes, of course). I spoke to one of the Engineers and he told me no problem. They upgraded my cable box to a Motorola SURFboard Model: SBG6580, it comes with 4 ports. The cable guy came and set up the modem in about 15 minutes. I have 2 routers and I was able to use two of the ports and plug both of my routers in. I hooked up my kids devices to the router with the parental controls and my other router has the DNS setup for Unblock-us DNS for my devices.
Let me know if you have any questions.

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