Network wiring notes - 8P8C / RJ45

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For other network related things, see:


What you were probably looking for: Ethernet

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):

                                                       Used by       Used by
  Color (568B)         Pin          Color(568A)        10/100Mbit    1Gbit/10gBit

  Orange-white         1            Green-white        ✓             ✓
  Orange               2            Green              ✓             ✓
  Green-white          3            Orange-white       ✓             ✓
  Blue                 4            BlueBlue-white           5            Blue-white                       ✓
  Green                6            Orange             ✓             ✓
  Brown-white          7            Brown-white                      ✓
  Brown                8            Brown
Removed insulation, ordered wires to 568B (right),
cut for equal length (not shown),
insert into plug (left)
Wires go to the end (can see wires, even its copper reflecting) (left).
Then use the crimping tool (not shown)

You typically want to make a straight cable, a.k.a. patch cable, which is 568B-to-568B.

(The other type is a crossover cable (568A-to-568B). There were useful before gigabit when you wanted to directly connect two NICs without a switch)

(568A-to-568A is functionally the same as a patch cable, but just confusing)

When wiring plugs

  • Cut the outer insulation (that holds the cables together)
  • Untangle the wire ends, so that they won't pull back when crimped
for ≥1gBit it is suggested that you keep the untwisted length as small as possible, to minimize near-end crosstalk[1]
  • order them according to the colors you want
  • cut them to equal length (if more than slightly different)
  • insert into plug (you could check the order again)
  • check that all the wires go to the end (shinyness seen from the tip is a good sign)
  • use the crimping tool, which shoves the pins into the wires, and fixes the wire in the plug
  • (optional: put some solid glue in the socket, to spread force when yanked)


  • technically, there are different plugs for braided and solid-copper wiring, that clamp/pierce the wire in different ways (see e.g. stranded solid contacts)
most of the time you'll work with solid
(in home use you can often get away with mixing this, but)

  • other things that may be using 8P8C include
RS-232 (varied pinouts)

...and many others

More notes on ethernet wiring

On standards

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).

On coexistence

  • 10Mbit and 100Mbit networking use only pair 2 and 3 (orange and green) in a 568-style cable
Countries using analog phones with 4P or 6P plugs (in 8P sockets) can have analog phones and FE coexist, because they use only pair 1 (blue, in the center). Large setups can do this to make their wiring simpler
It's also sometimes used for the (somewhat riskier) DIY variant of power over ethernet]
on-the-cheap two-pair cables will work in most places, but only because the NICs fall back to 100MBit
  • 1GBit and 10gBit ethernet uses all four pairs in a 568-style cable, so can't do the above trick,
  • You want cables rated Cat5e or better for 1gBit, Cat6 for 10gBit
so you can put in the wiring for whatever highest speed you expect to use in the next X years, and can do the actual speed upgrade later by replacing only the switch

On loopbacks

Loopbacks connect a port to itself.

You would want to do so to see whether the link light lights up. It's useful to check

  • whether a patchbay was wired properly
  • whether a wallplug or the cable to the nearest switch is broken
  • whether a (long) cable is broken
  • whether a switch/router port is broken

Connect: (If you're wiring a plug as a loopback, make sure you're not confused about which end is pin is pin 1)

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

You could take an existing cord (e.g. one that has one the retainer clip broken), cut it in half, and connect:

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

(This works for both 568A or 568B)

gBit loopback is a limited concept:

Gigabit NICs have crosstalk detection (detects how much signal interferes onto other wires), and will often decide that a loopback cable/plug is actually an extreme amount of crosstalk - any may not show link.

...defeating the point, ans you don't know lack of link-light means unless you know you have disabled crosstalk detection on the NIC/switch. -->

Other 8P8C wiring




More technical

On crossover cables

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)
    • informally refers to Cat5a
  • Cat5a
    • Currently still quite common
    • designed to support 100 MHz signals
    • e.g. (originally) 100mBit at <100m
    • but e.g. decent cable can do 1gBit at <100m and
  • Cat6
    • 100mBit, 1gBit at <100m
    • designed to support 250MHz signals
    • e.g. <55m for 10gBit (less if many are bundled)
  • Cat6a
    • designed to support 500MHz signals
    • e.g. 10GBit at <100m
  • Cat7
    • designed to support 600MHz signals
    • stricter about crosstalk (pairs individually insulated)
  • Cat7a
    • designed to support 1000MHz signals


  • Any cabling that is not shielded will crosstalk
with itself (hence pair shielding, see below)
with others, meaning permissible distances are lower when many are bundled (relevant to company wiring)
  • length of a single cable seems to be restricted more by propagation speed than e.g. category specs
hence commonly seeing ~100m regardless of exact cable/speed. It is a generic figure to keep in mind.
  • cable quality is usually more about cheap cables that don't actually conform
So a decent brand may matter more than category specs
  • category specs do become more relevant at higher speeds
bringing in some tradeoff between speed and range
  • shielded cable may in some environments make a larger difference than category
e.g. when you're putting it close to (significant) power lines
  • Often called copper, but need not be pure.
eg. CCA for Copper Clad Aluminium, CCS for Copper Clad Steel
which needs to be a little thicker to meet the resistance-per-length requirement
and may indicate lower quality in general - you may want to avoid it (verify)

In practice

Companies may want Cat6 (or perhaps Cat6a) for future compatibility, to be able to use 1gBit and 10gBit now (also somewhat assuming that the next big speed upgrade is still over copper TP).

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.

Home LAN can often still get away with Cat5a for gBit, but if you're pulling new wires, you may as well go for Cat6 (little price difference now, good for the next switch/speed upgrade or two).

On cable shielding

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)

UTP (Unshielded Twisted Pair) is typical in home use, though some variants like F-UTP (foiled whole) is not much more expensive so are also seen.

In longer-cabled setups, crosstalk within and between cables becomes relevant. Around powerful electronics you may receive enough interference. In both cases you want shielded versions to avoid large headaches later.

Earlier three-letter acronyms aren't entirely consistent, in part because the naming first got confusing, got used inconsistently, and got changed. Basically, anything with three letters (except UTP) is potentially not what you think.

The more-letter variants (look like U/STP or U-STP) are clearer. The parts:

  • overall shielding type
    • U - unshielded
    • S - braided shield
    • F - overall foil
    • SF - braid and foil
  • - or /
  • pair shielding type (to reduce crosstalk, common on long cables, 10gBit cables(verify))
    • U - unshielded
    • F - foil around each pair
  • TP (twisted pair, which is the subject)


  • prefabbed patch cables can make assumptions (e.g. that certain shielding is not relevant when short and probably not bundled) which you can't make on long spools made to wire a building.
  • When planning for higher speeds (e.g. 10gBit), it's sometimes handier to go slightly overkill
not because it's typically necessary, but because you may avoid some specific problems.
  • The common types may be U-UTP, F-UTP, U-FTP, S-FTP, SF-UTP (verify)
  • To make things even more interesting, there are 'industry standard' names that looks imilar but are different
some are pretty clear, e.g. UTP is U/UTP
others are fuzzier, e.g.
STP can refer to U/FTP, F/UTP, S/UTP, SF/UTP
SSTP can refer to S/FTP, or to SF/FTP
SFTP can refer to SF/UTP, S/FTP, SF/FTP
see e.g.

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 in Ethernet, while RJ45 actually refers to a specific telephone wiring on the 8P-style plug (probably the most common one among several), 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)

Plugs may have fewer actually present conductors than they have positions, so 8P2C, 8P4C, 8P6C, 8P8C, 6P2C, 6P4C, 6P6C, 4P2C, 4P4C all exist.

When there are less connectors than positions, they are in the middle positions; RJ-style wiring is from the middle out.

For most of us, the is interesting only in that you can plug a phone with 6P plugs into a 8P (ethernet-plus-phone) socket and have the phone work - the clip aligns the plug in the middle.

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)}}

There are exceptions to the 'always start in the middle', but they tend to be intentionally working around RJ-style wiring.

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