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I have two computers side by side (one work and the other personal). Due to the location of the wireless connection, sometimes the signal is weak, so I normally access one of the computer via the ethernet, but my modem has only a single jack. Is there a splitter or something that will allow me to access both computers at same time without having to swap the ethernet from computer to computer

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I'm missing something here: You say that the modem has only one port but you also apparently have a wireless router. How is it attached? Doesn't the router have any ports? –  Daniel R Hicks Sep 3 '12 at 13:36
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5 Answers

There's two options here - get a router or switch and connect it to the modem - if your modem is also a router, you'd have to switch off DHCP on the router. You might also be able to use a hub, but those are nearly extinct these days.

You might also be able to use a windows PC as a router by setting up Internet connection sharing if your modem is a 'simple' modem with no routing capabilities. I am unsure if you can turn off DHCP on ICS tho.

Splitters are passive devices, and ethernet as we know it uses a star configuration, so such a device is impossible.

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Strictly speaking, Ethernet uses a bus configuration, not a star. The wiring is configured in a star because modern Ethernet network always consist of a set of twisted pair connections with a hub or switch in the center. (In most home networks, the switch is built into a router.) In the bad old days, before twisted-pair Ethernet and cheap switches, a network often consisted of a single coaxial cable running past all systems on the network; most system would connect with a "tap", which included a spike driven into the cable. –  Isaac Rabinovitch Sep 3 '12 at 3:11
    
Keep it simple: get a router and don't turn off DHCP. Just install the router, connecting its uplink (or port 1, most routers these days auto-detect which port is the upstream connection) port to the modem (you may need to power cycle your modem) and connecting your two computers, using separate cables, to separate jacks on the router. Easy peasy. –  music2myear Sep 3 '12 at 3:12
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For clarity, there is such a thing as ethernet spitters. We use them here on sites when we're low on ports. They allow 2 devices to use a single run of network cable. You need one splitter at each end and it uses 2 of the twisted pairs per port. Unfortunately some devices (printers, VOIP phones) can have issues with this - embedded LAN often needs to be set to run at half-duplex in such a configuration. But it's still better than wireless. –  Beeblebrox Sep 3 '12 at 4:12
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No, you don't have to switch off DHCP on a router that's connected to another router. Any decent router can be both a DHCP client and DHCP server. –  Isaac Rabinovitch Sep 3 '12 at 7:41
    
Yes, a PC can serve as a router, but only if it has enough Ethernet ports. Unless you have the necessary hardware lying around, and don't mind the power costs of keeping the PC running all the time, it makes more sense to just buy a router. –  Isaac Rabinovitch Sep 3 '12 at 7:43
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Yes there is such a device, we call them RJ-45 splitters but they are also called RJ45 CAT 5 6 LAN Ethernet Splitter Connector Adapter. All three ends need to be female, the end with 1 female end will go to the cable that is connected to your modem/router, the end with 2 female ends go to each computers. Your done run one cable with splitter for 2 computers. They have this at Dollar Stores, eBay for $1.00, Radio Shack $10 I think you get the picture. Good luck

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That would allow the OP to physically connect all three. But just a physical splitter will not allow both work and personal computer to use the network at the same time. (Powering off one computer and then powering up the other computer would work). –  Hennes Sep 3 '12 at 0:58
    
@Hennes -- You don't understand -- it breaks the 8-wire cable into two 4-wire cables. –  Daniel R Hicks Sep 3 '12 at 1:03
    
That works for 10mbit and 100mbit connections if you use plugs at both ends. The OP's modem only has a single Ethernet plug. –  Hennes Sep 3 '12 at 1:14
    
For sharing connections you want a router or switch, not a splitter. –  music2myear Sep 3 '12 at 3:09
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I think the thing you're looking for is called a network switch. This is a data-link (second) or higher layer device for combining Ethernet connections. Actually a lot more functionality than you need, but some layer 2 switches are so cheap (NewEgg has them for $10) it doesn't matter.

There used to be simpler devices called network hubs that worked on the physical (first) layer. Switches got so cheap, they drove hubs off the market. Some people call switches hubs, but I'm a technical writer, and thus a semantic nitpicker.

There are ways to split Ethernet cables so that you don't need a hub or switch, but unless there's some big reason you can't install that extra box, splitting cables is an unnecessary hassle.

Here's why I'm not 100% sure I understand your question. It's not clear to me exactly what kind of device you're talking about when you refer to a "modem". Is the WAN interface configuration built into your "modem"? Or do both computers contain the configuration? If you're not sure, posting the make and model of this device would be helpful. If the "modem" does handle the WAN connection itself, than it's providing a network endpoint, and a switch or hub will work. Otherwise, you need to move a level up from switches and get a router. Since you're using physical Ethernet, the router doesn't have to be wireless, though you probably won't save any by buying a wired-only router for a two-computer network.

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Assuming your Ethernet is connected via a RJ45 plug:
No. A simple splitter will not work. You can use a switch or a hub though.

(RJ45 mentioned because you can do what you want if you used old coax to connect. Feel free to google for 10base2_t, but this is very unlikely)

enter image description here

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10B2! How quaint! –  Isaac Rabinovitch Sep 10 '12 at 4:12
    
Aye. Truly ancient. But there still are devices alive which use it. Even if my last coax switch died. (And yes, I meant a switch, not a hub. Back in the days of yore it cost as much as a small car and it was quaint enough that I saved it from the trashcan). –  Hennes Sep 10 '12 at 11:29
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The typical ethernet cable has 8 wires. Most ethernet schemes only use 4. Thus you can use a "splitter" at each end to effectively squeeze 2 cables into 1. There are several varieties available.

(But note that some "enterprise" wiring used 4 or 6-wire cables, so this scheme won't work with those. Also, the scheme won't work with certain high-speed ethernet protocols that use more than 4 wires.)

The other option is a small hub, but this will not work with some routers that assume one computer per port.

Added: And people in the past have actually gotten away with "OR-tying" two Ethernet cables together, relying on Ethernet collision detection to handle conflicts. But this is not a very reliable approach, if it even works at all anymore.

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This is a horrible idea. Yes, at least in the past, you could use a splitter to accomplish this. I dont know if this holds true. However, the two PCs would not be able to communicate with each other, since the TX/RX are not switched. –  Keltari Sep 3 '12 at 3:27
    
IIRC, Gigabit Ethernet requires all 8 wires to be available? –  grawity Sep 3 '12 at 9:35
    
@Keltari -- Which is a bad idea? Splitting the cable is perfectly fine so long as the protocol uses no more than 4 wires. And the OR-tying trick I didn't recommend, just said it had been done. –  Daniel R Hicks Sep 3 '12 at 13:32
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