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Understanding Ethernet

Before you get started you need to know a little about how an ethernet network works. Out of the eight wires used in 10Base-T cable (known also as ethernet or twisted pair), only 2 pairs are needed. There is a transmit (Tx) and a receive (Rx) pair (pins 1&2 and 3&6). In most cases you would use a straight through cable (both ends look the same, also known as a patch cable), however there are occasions where you would have to use a crossover cable (pins 1&2 and 3&6 are swapped at the other end). The picture below shows what the ends of a crossover cable should look like (seen from below) when held up to each other. I painted in the wires so you can see them better in the picture. NOTE: The colors in your cable may be different, but it doesn't matter as long as they are paired correctly.

A straight through cable is used when you connect a computer to a hub, switch, or router. A crossover cable is used when you connect a hub or switch to a router or bridge (as in the dsl modem). A straight through cable will act as a crossover cable (and vise versa) when plugged into an uplink port (found on most hubs and switches). If you plug a crossover cable into a port that's designed for a straight through cable (and vice versa) then you won't get a link indicator light showing on the device, therefore causing the connection not to work. Keep this in mind, burn it in if you have to, because it's VERY important that you use the right cable for the job or nothing will work. See the table below for a quick reference on the cable to use:

D E V I C E S	DSL Modem	Computer	Router WAN	Router LAN	Hub/Switch
DSL Modem	- - - - -	Patch	Patch	- - - - -	Crossover / Uplink
Computer	Patch	Crossover	Crossover	Patch	Patch
Router WAN	Patch	Crossover	- - - - -	Patch	Patch
Router LAN	- - - - -	Patch	Patch	- - - - -	Crossover / Uplink
Hub/Switch	Crossover / Uplink	Patch	Patch	Crossover / Uplink	Crossover / Uplink

Most devices have an uplink port on them that share a regular one (indicated by a line drawn between the two). If you are using the uplink port on your switch or hub you might want to tape over the one that it shares to prevent you from accidentally plugging in another cable and effectively killing both ports. Remember: No Light, No Link (It won't work).

The switch shown below doesn't have an uplink port, so if you were going to connect it to your dsl modem you would have to use a crossover cable between any of the 5 ports and the LAN/10BaseT port on the DSL modem.

You may want to mark your crossover cables in some way to keep them from getting confused with your straight through (patch) cables. I normally use a little flag or zip tie on one end, but you can even use a marker or colored tape. Whatever works for you.

There are some router manufacturers that don't think it's worth the extra plastic or ink to indicate that there is an association between the uplink port and a standard one. Below is just one example of this. I had to paint the raised plastic so that you could read the writing. Some even think you read from right to left :)

They don't have anything there that would indicate to you that you can't plug something into both the uplink and port 1 at the same time. If you were to plug a cable into the uplink port on this then neither that machine nor the one on port 1 would be able to get out on the net. HINT: This would be a good place for a little piece of tape.

Some nicer routers and switches (such as 3com, Intel, Kingston, and others) actually have a port that is able to be changed between an uplink or standard port with the push of a button or flip of a switch. It is also shown sometimes with an X for crossover or II for straight through. Again, remember that if there isn't a link light then it's not going to work.

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This site is designed to be of some assistance, however is not meant to be your sole source of information. By following the suggestions given on this page you understand that you do these things at your own risk. The steps mentioned here may or may not work for your purposes. There is no warranty or support of any kind provided by HAL-PC for the topics discussed in this site. The only support for your home network that is given is by me personally, at my choosing, on my own time, and via email only.
I know... I hate disclaimers too.