How to wire your own ethernet cables and connectors.
What You Need:
Required:

* Cable - bulk Category 5, 5e, 6 or 7 cable
* Wire Cutters - to cut and strip the cable if necessary
* For Patch Cables: RJ45 Plugs
* RJ45 Crimper
* For Fixed Wiring: RJ45 Jacks

Recommended:

* Wire Stripper
* Cable Tester

About the Cable:
You can find bulk supplies of the cable at many computer stores or most electrical or home centers. You want UTP (Unshielded Twisted Pair) Category 5 cable for basic 10/100 functionality. You want CAT 5e for gigabit (1000BaseT) operation and CAT 6 or 7 gives you a measure of future proofing. Bulk cable comes in many types, there are 2 basic categories, solid and braided cable. Braided cable tends to work better in patch applications for desktop use. It is more flexible and resilient than solid cable and easier to work with, but really meant for shorter lengths. Solid cable is meant for longer runs in a fixed position. Plenum rated cable must be used whenever the cable travels through an air circulation space. For example, above a false ceiling or below a raised floor. It may be difficult or impossible to tell from the package what type of cable it is, so peal out an end and investigate.

Here is what the internals of the cable look like:


Internal Cable Structure and Color Coding

Inside the cable, there are 8 color coded wires. These wires are twisted into 4 pairs of wires, each pair has a common color theme. One wire in the pair being a solid or primarily solid colored wire and the other being a primarily white wire with a colored stripe (Sometimes cables won't have any color on the striped wire, the only way to tell which is which is to check which wire it is twisted around). Examples of the naming schemes used are: Orange (alternatively Orange/White) for the solid colored wire and White/Orange for the striped cable. The twists are extremely important. They are there to counteract noise and interference. It is important to wire according to a standard to get proper performance from the cable. The TIA/EIA-568-A specifies two wiring standards for an 8-position modular connector such as RJ45. The two wiring standards, T568A and T568B vary only in the arrangement of the colored pairs. Tom writes to say "...sources suggest using T568A cabling since T568B is the AT&T standard, but the US Government specifies T568A since it matches USOC cabling for pairs 1 & 2, which allows it to work for 1/2 line phones...". Your choice might be determined by the need to match existing wiring, jacks or personal preference, but you should maintain consistency. I've shown both below for straight through cabling and just T568B for cross over cabling.
About RJ45 Plugs and Jacks:
The RJ45 plug is an 8-position modular connector that looks like a large phone plug. There are a couple variations available. The primary variation you need to pay attention to is whether the connector is intended for braided or solid wire. For braided/stranded wires, the connector has sharp pointed contacts that actually pierce the wire. For solid wires, the connector has fingers which cut through the insulation and make contact with the wire by grasping it from both sides. The connector is the weak point in an ethernet cable, choosing the wrong one will often cause grief later. If you just walk into a computer store, it's nearly impossible to tell what type of plug it is. You may be able to determine what type it is by crimping one without a cable.

RJ45 jacks come in a variety styles intended for several different mounting options. The choice is one of requirements and preference. RJ45 jacks are designed to work only with solid cable. Most jacks come labeled with color codes for either T568A, T568B or both. Make sure you end up with the correct one.

Here is a diagram and pin out:


RJ45 Plug and Jack Pin Out

Ethernet Cable Pin Outs:
There are two basic cable pin outs. A straight through cable, which is used to connect to a hub or switch, and a cross over cable used to operate in a peer-to-peer fashion without a hub/switch. Generally all fixed wiring should be run as straight through. Some ethernet interfaces can cross and un-cross a cable automatically as needed, a handy feature.

Standard, Straight-Through Wiring (both ends are the same):

Straight-Through Cable Pin Out for T568A

Cross Over Cable (T568B):

Cross Over Cable Pin Outs

+Note: The cross over cable layout is suitable for 1000Base-T operation, all 4 pairs are crossed.

I don't see too many Giga Cross overs as the cable is usually Cat6 and a pain to work with. Might be a point to note that a standard Cat5 cross over is simply T568A standard at one end and T568B at the other. One other thing we have seen a bit of is the ability of unshielded Gigabit cabling getting into UHF radios. One such fault was only removed by replacing a Gigabit switch with a 100Mb switch. As a general rule for my clients - if they want Gigabit they should use fibre, and faster is not always better - telemetry for example (or SCADA) works better using a smaller pipe.
Hope this info is of use to someone... Looks like Mr Green likes it anywho.

Just thought id post another diagram, outlines the standards a bit clearer.

Probably biggest point to remember is when doing terminations is try to keep the sheathing short so the connector can go right over the sheath. This will help in two ways the cable won't be able to untwist which will cause crosstalk, if the cable is tucked into the connector the more untwist the slower the speeds on the cable. This is more important for high bandwidth networks but its best to get into the standard.