How do I split my modem connection into a router and an additional ethernet output?

I have my cable modem in one room. I have a ethernet wire going through the wall connecting to the router. I have an Office I want to run an ethernet cable to, which would be pretty easy to do if I can run the wire to the room w/ the cable modem. Unfortunately I can't run a wire from my router to the Office.

Is this possible with a splitter or switch or some other device?

Hopefully the image helps:

Solution 1:

The easiest option will be to move the router to where the modem is and put a switch where the router was by your TV and use the existing cable between the current modem and router setup to connect the router and switch. Then you can run a cable from the office to your router. If WiFi will be an issue with the change, you can add a wireless access point to your switch as well. If you add a access point, have the name and security settings be the same as your router.

Solution 2:

All cable modems in my experience will, on the user-accessible Ethernet-port side, only talk to the first device that connects to it or that responds to it. They will ignore all other devices until a reboot.

So even if you physically connect more than one device to the Ethernet port of a cable modem, which is possible if you use an Ethernet switch, it won't work.

FYI most routers have a built in Ethernet switch with ports labeled LAN 1,2,3,4. Some cable modems have built in routers and 4-port switches.

You can do this:

• Move existing router next to cable modem (call this the first router).
• Disable wireless on this router.
• Obtain a second router and configure like so:
  • Connect nothing to second router's WAN port.
  • Assign router IP in same subnet as first router, with an IP that won't get affected by first router's DHCP range.
  • Connect LAN port of first router to LAN port of second router.
  • Disable DHCP on second router.
  • Disable routing on this second router (you want only the wireless) if this is possible.
  • Configure wireless as you like on second router.
  • If you have wired devices near where you want the wireless to originate from, you can still connect them to the LAN ports of the second router, and it will work.

I have a similar setup, the "second router" in my case is an Asus RT-AC56U (256MB dual core unit with a big heatsink over the chipset, running OpenWRT derivative) which has a convenient "Access Point" setting that didn't require messing with a lot of settings after the fact. Any router with wireless that can be configured above should work though.