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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:

Diagram of desired layout

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    Unless you move the router to be near the cable modern, I don’t see how this can happen.
    – Giacomo1968
    Jan 16, 2019 at 0:09
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    Why do you care where the router is? What's preventing you from moving it to where the cable modem is? Is it WiFi coverage? Needing wired ports there? Or what?
    – David Schwartz
    Jan 16, 2019 at 0:16
  • Seems like a simple solution would be to buy a new router and put it in the room with the modem.
    – Scott - Слава Україні
    Jan 16, 2019 at 0:21
  • @DavidSchwartz -yes, the router is in a good central location now. Diagram is not to scale
    – joel
    Jan 16, 2019 at 0:31
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    Do you actually get multiple public IP addresses from your ISP? If not, then this whole setup won't work at layer 3.
    – chrylis -cautiouslyoptimistic-
    Jan 16, 2019 at 4:31

4 Answers 4

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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.

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    You can also use a second router as a combination switch and access point. Just don't use its WAN port and disable its DHCP server. Assign it an IP address inside the main router's LAN IP block but outside its DHCP range.
    – David Schwartz
    Jan 16, 2019 at 0:33
  • Of course. I was going the easiest route with the least amount of configuration needed.
    – HazardousGlitch
    Jan 16, 2019 at 0:35
  • OK, thanks. My only question is how would I add a wireless access point to the switch? Do you have a hardware recommendation for a wireless access point? The extender is OK but it's kind of annoying having to switch to the _EXT connection
    – joel
    Jan 16, 2019 at 0:35
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    Do not use a wireless extender. Just use either an access point or a second router acting as a switch and access point. You can use pretty much any router with the choice depending on your budget, port requirements, and WiFi requirements. For example, an Archer C50 (my first hit googling "cheap dual band router") is about $40 and would give you dual band WiFi and three wired ports (because you can't use the WAN port and need to use one port to connect to the main router).
    – David Schwartz
    Jan 16, 2019 at 0:36
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    I think your comments are worthy of an answer @DavidSchwartz
    – HazardousGlitch
    Jan 16, 2019 at 0:54
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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.

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    Why disable wireless on the first router?
    – David Schwartz
    Jan 16, 2019 at 0:26
  • It took me a second: you're talking about a passive splitter - I've never even seen one of those.
    – Mazura
    Jan 16, 2019 at 14:31
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    Passive splitting is only possible on ancient thicknet/thinnet network cables of the 80's. It still wouldn't work in this scenario.
    – LawrenceC
    Jan 16, 2019 at 14:34
  • Per disabling the wireless, it will likely work OK if SSIDs are the same, but eliminate that as a potential problem source during setup.
    – LawrenceC
    Jan 16, 2019 at 17:48
  • That's a great solution : switch back to BNC :)
    – Mazura
    Jan 16, 2019 at 20:04
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Check if there are any Ethernet wall sockets in your office and close to your router, if there aren't then install one.

What I would do is install Ethernet wall socket close to the router and room which has devices I want to connect to the router and just slid simple fiber cable through it.

Basically you could just drill a hole to the roof above your router and slid ethernet cable upstairs that way, but it looks ugly as hell

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While the other answers are the "right" way to do it, they require some networking know how to get them working correctly.

If you want something that "just works" and you're willing to take a small performance hit on the new link, you can just buy a pair of powerline Internet modules.

Plug one into a wall power outlet near your router and run an Ethernet cable from it to one of the ports on your router. Plug the other one into an outlet in the "new" room near your other equipment.

If you just have one device in the new room, then you can run an Ethernet cable from the module directly into the Ethernet port on the device.

This will work without any configuration unless the two sections of the house are electrically isolated in some way - usually not the case. But don't plug either of the modules into a power strip, etc. which has an electrical noise filter in it. That could filter out the Internet signal.

If you have more than one device in the new room, then you will have to install a switch between the second module and the devices. That should be pretty simple to do, but I have never worked with switches, so I don't know how to configure them.

You might also be able to get additional powerline modules if you just need another port or two. I know this can be done, but I have never tried it. I have also seen at least one model where the second module had more than one Ethernet port on it.

I'm writing this with my notebook hooked up to a 500Mbs powerline Internet adapter because my internal WiFi card is dead. It works great, but I don't play any online games or do anything else which requires maximum bandwidth.

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