How to Network Two Computers with Just One Wire

So your buddy brings his computer over to play a LAN game, but you don’t have a switch. You could waste a lot of time looking for a switch (time which could have been spent playing), or you could just use one wire to connect the two computers together. To be able to do this, we first need to understand the RJ-45 pinout and the reason that switches are normally used to connect two or more computers.

There are two specifications for RJ-45 pinout: T-568A and T-568B. This is true for normal cables and crossover cables. However, the wiring of crossover cables will be different than the wiring for normal cables (also known as straight through cables) regardless of which specification you use. The point of using these specifications is so that other technicians who see your wiring wont be confused. If you’re doing a single crossover cable for your personal use, the specification isn’t going to matter. As long as you put the wires in the right places in each connector, which color goes where is a non issue. However, for the purposes of this article I will need to follow one of the specifications. The pinout of the two different specifications is as follows:

T-568A
1. Green & white
2. Green
3. Orange & white
4. Blue
5. Blue & white
6. Orange
7. Brown & white
8. Brown

T-568B
1. Orange & white
2. Orange
3. Green & white
4. Blue
5. Blue & white
6. Green
7. Brown & white
8. Brown

Wait, why aren’t the color pairs matched up?

To put it simply, not every wire is used for most network cables. In fact, only four are used. Here is the layout of of data transmission:

  1. TX+
  2. TX-
  3. RX+
  4. Unused
  5. Unused
  6. RX-
  7. Unused
  8. Unused

So how does this apply to me?

The idea behind making a crossover cable is that the two transmission lines of one end connect to the two receiving lines of the other end and vise-versa. So far I have only listed one end of the specification. If we were making a straight through cable, you would use the same color scheme on both sides. However, since what we want is a crossover cable, we need to rearrange the wires on one side of the cable.

T-568A (Crossover)
1. Green & white ———————> Orange & white
2. Green ——————————–> Orange
3. Orange & white ———————> Green & white
4. Blue ————————————> Blue
5. Blue & white ————————-> Blue & white
6. Orange ——————————–> Green
7. Brown & white ———————–> Brown & white
8. Brown ———————————-> Brown

T-568B (Crossover)
1. Orange & white ———————> Green & white
2. Orange ——————————–> Green
3. Green & white ———————–> Orange & white
4. Blue ————————————> Blue
5. Blue & white ————————-> Blue & white
6. Green ———————————-> Orange
7. Brown & white ———————–> Brown & white
8. Brown ———————————-> Brown

But why can’t I just use a regular cable?

Your network interface card (NIC) is built to send and receive data by the standards I laid out earlier in the article. Every other NIC is built to send and receive data in the same way. Data sent to another computer directly through a straight through cable would never be received because the pins that transmit on your computer would be directly connected to the pins that transmit on your friend’s computer. Either the data would arrive at a pin that wasn’t listening (because it was built to transmit, not receive), or it would collide with data being transmitted from the other computer.

The reason that straight through cables work with a switch is that the switch behaves as a crossover adapter. It takes data sent to it from one wire and sends data to the right pins on another wire. When you make a crossover cable, you’re essentially doing the same thing manually. While it would be difficult (not to mention inefficient, more expensive, and beyond the scope of this article) to hook up three or more computers with crossover cables instead of using a switch and straight through cables, hooking up just two computers with a crossover cable is easier and cheaper than using a switch and straight through cables.

I made the cable and connected the computers, but they still can’t talk to each other

If your computer is usually hooked up to a switch or router, it is likely that your network connection is set to automatically receive an IP address. In peer to peer networking, there is no DHCP server to give you an IP address. Therefore, you are going to have to manually define your own. I recommend the following setup:

Computer 1:
IP address: 192.168.0.1
Subnet mask: 255.255.255.0
Default Gateway: 192.168.0.1

Computer 2:
IP address: 192.168.0.2
Subnet mask: 255.255.255.0
Default Gateway: 192.168.0.1

Once everything is setup, you should be able to play a LAN game or transfer files easily. It wont matter which end of the cable is plugged into which computer. If you still have issues communicating with the other computer, you may want to disable your firewall temporarily. Keep an eye out for my next tutorial, where I describe how to set up a larger home or small business network.

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