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I've thought cat5(e) means always RJ45 lan cable. But my house has "cat5e 2-pair rj11 green (data) cables" from the living room to each room. My internet router is in my living room in the 1st floor (XFinity). What I want to do is to connect the Internet line from my router through the green cat5e rj11 cable to two upstairs rooms. My problem is how to connect from my router(RJ45) to the port RJ11 on the wall of living room and from the port RJ11 in the room to my other Internet devices (RJ45).

Is there any easy adaptor available?

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  • Thank you everyone who added comments and answers. FYI, it turned out that the cable inside the wall has 4-pairs and the jack was RJ11. brown and white brown were disconnected from the jack. I will replace the jack with RJ45. I don't know why the technician who installed it used RJ11 jack at that time.
    – Daebarkee
    Apr 25, 2016 at 14:02
  • Because almost no one uses the cables in the wall for Ethernet. Even if they were wired with RJ45 jacks, most people wouldn't know what to do with them. But nearly everybody understands how to plug in a phone.
    – longneck
    Apr 25, 2016 at 15:33
  • In that case, connect all 4 pairs, of course.
    – Ecnerwal
    Apr 25, 2016 at 15:43

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The best solution you can arrange for 2-pair cables will be limited to 100 Mbits/second (or 10 times slower than the current most common standard wired ethernet speed.)

Rather than looking for an adapter (dubious at best - any you find will likely be intended for telephone signals rather than data), I'd suggest just removing the RJ-11 jacks, and replacing them with Cat5e RJ-45 jacks.

You'll connect your two pairs to the "Orange & White/orange and Green & White/green" marked terminals (1,2,3,6) and ignore the blue/brown marked terminals (4,5,7,8)

Edit: Since the cables have been discovered to be normal 4 pair cables after all, connect all 4 pairs, and Gigabit (1000 Mbits/sec) speeds should be possible across them.

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  • You had it right until the last sentence. When terminating you should terminate the jack as it should be according to standards not ignore any pairs. That way in the future any use of that jack and cable will comply with EIA/TIA/ANSI standards. TIA 568B pattern is the most popular pattern.
    – ArchonOSX
    Apr 25, 2016 at 10:52
  • @ArchonOSX - Daebarkee only HAS 2 pairs in the existing cables. Regardless of what colors those pairs are, they need to be connected to the RJ-45 in the positions occupied by orange and green or it will not work at all (as opposed to working, but only at 100 Mbit.) If new cables are pulled, that would be a different matter,but the question was how to make use of the exsiting 2-pair cables. Without 4 pairs, they cannot be "wired to the standard" that is based on 4 pairs.
    – Ecnerwal
    Apr 25, 2016 at 13:00
  • Ahhh yes I misunderstood the OP. I thought maybe someone had just cut off the other pairs to terminate it to the RJ11 jack. The previous owner going to the trouble of running Cat5e cable and then only installing 2 pair wire seems pretty silly.
    – ArchonOSX
    Apr 25, 2016 at 16:38
  • Happily, it seems that the OP was mistaken and actually has normal 4-pair cables, not two pair as described - so connecting all the wires and getting gigabit speeds should be possible.
    – Ecnerwal
    Apr 26, 2016 at 1:57
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So, it's a dirty little secret that most ethernet twisted pair wiring uses only 2 of the 4 pairs -- at least until recently. But, 4 pair is better for a lot of reasons -- like power over ethernet, redundancy in case of a broken wire, etc. Also, some old ethernet twisted pair used all four pairs.

As to your question about an adapter -- dunno on that one. An adapter CABLE that had an RJ-11 on one end and RJ-45 on the other would do the job. You'd need to make sure that there are pairs that end up connected to pins 1&2 and 3&6.

Sorry I can't point to a specific adapter. Seems like there would be. Or maybe you can replace the RJ-11 connectors with RJ-45?

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  • This is factually dubious at best. IEEE 802.3ab has been around since 1999, which makes the "use of all 4 pairs" a common practice for the last 17 years; nor does this answer mention that using only 2 pairs will limit speeds to 100Mbits/second rather than 1000Mbits/second.
    – Ecnerwal
    Apr 25, 2016 at 2:32
  • Apparently it is an open secret. You last sentence was the answer.
    – ArchonOSX
    Apr 25, 2016 at 10:57

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