Network wiring notes - 8P8C / RJ45

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What you were probably looking for

T568A/B (10-BASE-T and 100-BASE-TX):

With pin positions are counted from left to right with the contacts facing you (clip on the back) and pointing up (cable coming out the bottom):

 Color (568B)         Pin          Color(568A)
  Orange-white         1            Green-white        
  Orange               2            Green
  Green-white          3            Orange-white    
  Blue                 4            Blue
  Blue-white           5            Blue-white
  Green                6            Orange                                 
  Brown-white          7            Brown-white
  Brown                8            Brown

Cut the outer insulation and order the wires (left), cut for equal length, and insert into plug (right)
Check that the wires go to the end of the plug

100Mbit ethernet (Fast Ethernet, FE) uses only two pairs, so you could use a wire for more than just that - e.g. two links, or one link and a phone, or some other combination.

Specifically, it uses pins 1, 2, 3, and 6, which in the standard wiring are the orange and green wire pairs.

Gigabit ethernet uses all four pairs.

On crossover cables

Given the two plug colorings in 568-type wiring, cables wired with these can be:

  • A straight cable, 568B-568B (or the functionally equivalent 568A-568A, but the B-B variant seems to be used everywhere, probably to avoid color confusion)
  • A crossover cable (also patch cable) has 568A on one end and 568B on the other. (crossing is something effectively done by a switch or hub, so you use straight cables unless you don't use switches. For end users they can be useful for direct computer-computer connections).

Gigabit ethernet doesn't need crossovers -- it decided to handle that case inside the NIC rather than have you do it the cable. You use straight cables everywhere (NIC-switch-NIC and NIC-NIC).

Gigabit crossovers exist

On Loopbacks

Loopbacks connect a port to itself. This can be used to test whether a long cable is broken, and whether a switch/router port is broken (or perhaps dirty or corroded), both just by seeing whether the link light comes on.


Pin 1 to 3
Pin 2 to 6
Pin 4 to 7   (for a gBit loopback)
Pin 5 to 8   (for a gBit loopback)

(If you're wiring a socket as a loopback, know for sure which pin is pin 1).

To create a loopback from a plug-with-cable you cut (that was wired according to 568A or 568B), this means:

Orange-white  to  Green-white 
Orange        to  Green
Blue          to  Brown-white  (for a gBit loopback)
Brown         to  Blue-white   (for a gBit loopback)

On gBit loopback:

Loopback plugs made in the 10/100 era don't have the connections mentined for gBit above.

If you wire the plug that way, should also know that gBit loopbacks are only useful reliable for testing on a network card on which you can disable crosstalk detection (a standard gBit feature which detects how much signal interferes onto other wires), because use of a loopback may make that feature decide there is horrible crosstalk, and will not show link.

More notes on ethernet wiring

The wiring used on 10Mbit, 100Mbit (specifically 10-BASE-T and 100-BASE-TX) ethernet over 8P8C (informally RJ45) plugs is defined by TIA/EIA-568-B, which define two plug wiring alternatives, 568A and 568B. Notice the lack of dashes; 568-B is the standard they are part of, 568-A a completely different standard (yes, that naming is stupidly confusing).

Note that both 10Mbit and 100Mbit networking use only pair 2 and 3 (orange and green) in the standard (The blue pair is pair 1, orange is pair 2, green is pair 3, and brown is pair 4.)

This means that Americans or anyone else using 4P/6P-style phone connectors can use fully wired cables (most are fully wired - relatively few (cheaper) cables are only two-pair for only Ethernet) to wire their house/company and have the same sockets be usable to plug in phones, a computer (or both with a trivial splitter). Various companies can use use this to make their wiring simpler.

Gigabit ethernet

GBit ethernet can use cables wired 568-style, preferably rated Cat5e, or better.

Specifically, you want straight wiring and four-pair cable. Most older cables are, so can be used at gBit speeds, as 1000-BASE-T uses all four pairs instead of just two.

(If you press your own plugs, it is suggested that you keep the untwisted length as small as possible, to minimize near-end crosstalk[1])

Some implications:

  • you can't do the phone/networking split mentioned above
  • on-the-cheap two-pair cables will work, but only because the NICs fall back to 100mbit
  • you can mix 10/100/1000 in your network, by replacing switches (handy for partial/gradual upgrades), without having to wire about the cabling. long as the cable is rated Cat5e (or better)

On cable standards (Cat5, etc.)

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

According to the specs:

  • Cat5
    • rarely seen - it has fallen out of favour and is barely sold
    • (...but your company may still be wired with it)
    • regularly and informally refers to Cat5a
  • Cat5a
    • Currently still quite common
    • 100mBit, 1gBit at <100m
  • Cat6
    • 100mBit, 1gBit at <100m
    • <55m for 10gBit (less if many are bundled)
  • Cat6a
    • 10GBit at <100m
  • Cat7 - stricter about crosstalk (pairs individually insulated)

Any cabling that is not shielded will crosstalk, meaning permissible distances are lower when many are bundled (relevant to company wiring), or be likely to get a lot of outside interference.

The Shiny Special Expensive cables sold in the sort of computer shops that only sell things that come in plastic boxes are generally not necessary, particularly not on the few-meter cables for your home LAN.

For example, Cat6 was made for 10gBit, most single computers don't have a source of data to actually use that speed, and even if they had, it's hard to get a 10gBit switch.

Companies may want Cat6 (or perhaps Cat6a) for future compatibility, to be able to use gBit now and assuming that nothing replaces copper before the next update.

Cat7 can go beyond 10gBit at short-ish distances, but chances are you won't be needing that any time soon. Only data centers might care.

Naming pendantics and telephony

When we say RJ45, we often mean something like "Ethernet wiring on an 8P8C plug."

RJ is the group of plugs that can be described by their positions and connectors, such as 8P8C for the plug used in networking (and in the US often for phones), RJ45 actually refers to a specific telephone wiring (probably the most common one, among several) on the 8P-style plug, while 8P8C refers to that plug itself and no specific wiring. Regardless, most people call the plug RJ45, regardless of wiring. (See also Common_plugs_and_connectors#Modular_connector_.28and_Registered_Jack.29.3B_.3FP.3FC)

Technically, plugs may have fewer actually present conductors than they have positions, so 8P2C, 8P4C, 8P6C, 8P8C, 6P2C, 6P4C, 6P6C, 4P2C, 4P4C all exist, where if there are less connectors than positions, they are always in the middle positions. (RJ-type wiring is from the middle out. If more more than the middle two wires are used in telephone wiring, they carry either power, or a second (RJ14) or even third (RJ25) telephone line on the same wire, but consumers rarely see this type of phone wiring)

This is interesting in that you can plug a phone with 6P plugs into a 8P socket and have the phone work - the clip aligns the plug in the middle.

The most common use of 6P outside of the US is probably phone wiring according to RJ11, which often use just a single pair in the middle. In the US, 8P connectors with the RJ45 phone wiring is common.

See also