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I have just moved into a new home and there appear to be ethernet lines running to several rooms, all converging in a wiring box. However, I can't figure out how to make this thing work properly. Here's how it's set up.

Home wiring box

The blue cables I'm holding in this picture apparently run to the other rooms of the house. The yellow cable goes to my WRT54G router. A cable modem is plugged into the router. Wireless networking is functioning properly.

If I plug my laptop directly into this box, and disable wireless networking, I can reach the router from the laptop, and the light for the yellow cable going into the router turns on. However, if I plug any of the blue cables into any of the ports on this box, the laptop loses connectivity and the link light on the router turns off, as though the router were not plugged into the box at all. Why does this happen?

The cables are cat 3e. There is an alarm system, but it is not active, and I don't think it's actually connected to the box at all. The top-left port is marked "RJ31X Line Intercept", and there is a jumper that can be plugged in there, but it doesn't seem to matter whether it is or is not plugged in.

How do I correctly use this box?

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"The cables are cat 3e." Do you mean Cat. 3 or Cat. 5e? I don't think there is such a thing as "Cat. 3e". – PleaseStand Sep 22 '12 at 2:04

This splitter is made for connecting up to ten analog telephones to plain old telephone service (POTS). It is not an Ethernet switch. You cannot split an Ethernet link by just electrically connecting multiple computers' network cards in parallel. This is why when you plug in the second computer, you lose your Internet connection.

When used as intended for your landline phone service, it brings in up to four separate telephone lines through the Telco In jack, then through the RJ31X alarm system jack, and then to all the Telephone OUT jacks, which are wired in parallel. (The splitter just happens to use the same kind of jacks Ethernet does.)

To connect your computers to the Internet, you should connect the cable from your cable modem into the WAN port of a home router and connect the cables from other rooms to the LAN ports. If you run out of LAN ports, use a real Ethernet switch to connect more computers. 100BASE-TX Ethernet switches are very inexpensive these days.

The bottom section of the box, by the way, is a normal 6-way passive cable TV splitter. The "-9 dB Output" part means that the signal from each output connection is about 1/8th as strong as the input signal, because 10(-9)/10 ≈ 1/8; it splits the signal in half three times.

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I think this box is for telephone wiring (since it ways telco in and telephone out). I guess all the sockets are actually physically connected together.

You need an Ethernet switch.

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As the other have stated, this is a telephone patch. In your photo, the upper left port is where you would plug in the phone line for an alarm system and it would be able to grab the line in case of an emergency where fire, police, or ambulance would need to be summoned by your security provider. At the bottom, you could take one of the cable outputs to your cable modem for internet access.

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