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What to typical network switches do with the Ethernet NC lines (pins 4, 5, 7, and 8)? I'm considering a few (non-standard, for personal use only) projects where it would be very useful to have those switched with normal TCP/IP data.

Edit: 100Mbps only. I know Gbit connections use all eight lines.

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NC stands for not connected – fahadsadah Jun 20 '10 at 16:02
up vote 3 down vote accepted

At 10/100Mbps, they aren't used for anything. In fact, I've made special cables that use those pairs for other data, just like you are suggesting. Note that if you do too much with them though, you risk electrical interference with the data carried over the pairs, so you need to be careful. But I've done things like split them out so that I can carry two 10/100 Ethernet runs, or carried voice over the other pairs (just make sure you don't wire up the pairs into the same jack that the ethernet is on, or you'll risk frying your NIC).

EDIT: Answering question in the comments: well, if you are running ethernet over the second two pairs, sure, it should be fine. If you are running something else, then probably not. If you are lucky things will just fail. If you are unlucky, you'll fry your switch or whatever other device you've got connected to the hacked pairs.

EDIT2: Evan prompted me to double check wht I was saying here, and I found this interesting section in the wikipedia article on Ethernet over twisted pair which also points out the bit about hubs short-circuiting the unused pair:

10BASE-T and 100BASE-TX only require two pairs to operate, located on pins 1 plus 2 and pins 3 plus 6. Since 10BASE-T and 100BASE-TX need only two pairs and Category 5 cable has four pairs, it is possible, but not standards compliant, to run two network connections (or a network connection and two phone lines) over a Category 5 cable by using the normally unused pairs (pins 4–5, 7–8) in 10- and 100-Mbit/s configurations. In practice, great care must be taken to separate these pairs as most 10/100-Mbit/s hubs, switches and PCs internally hardwire pins 4–5 together and pins 7–8 together, thereby creating a short-circuit across each "unused" pair. Moreover, 1000BASE-T requires all four pairs to operate, pins 1 and 2, 3 and 6 — as well as 4 and 5, 7 and 8.

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This was a common config at my old job. Combo data/voice runs to cubes. Made it easy, one wire, two devices. Just keep the patch-panels separated and there you go! Once we went 100Mb they had to rip a lot of it out as it was mostly Cat3. – sysadmin1138 Jun 20 '10 at 5:15
I'm curious as to if its possible to run such a custom cable through a normal network switch. – Alex S Jun 20 '10 at 5:51
Full-duplex Ethernet is over-rated, eh? – Evan Anderson Jun 20 '10 at 11:09
@Evan Anderson Actually, full duplex operation with 10BASE-T or 100BASE-TX still only uses two of the 4 pairs. – Jed Daniels Jun 20 '10 at 18:13
@Jed Daniels: You're quite right-- it's just transmitting and receiving simultaneously w/ CSMA/CD disabled. I don't know what I was thinking (other than not thinking) when I made that comment. – Evan Anderson Jun 20 '10 at 19:20

Those are used for data transmission of speeds higher than 100Mbps. All 8 wires are used in Gb and 10Gb connections.

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I'm aware. I should have indicated 100Mbps, my apologies. – Alex S Jun 20 '10 at 5:00

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