What Do Ethernet Lights Mean?
Ethernet ports on network devices like routers , switches , and network interface cards contain small LED lights…
RJ11 and RJ45 look similar enough that people try to use one in place of the other. They are different sizes, carry different signals at different voltages, and are wired to different standards. An RJ11 plug will physically enter an RJ45 socket — and that is the source of most of the trouble.
Quick Comparison
| RJ11 | RJ45 | |
|---|---|---|
| Correct name | 6P2C or 6P4C | 8P8C |
| Positions / contacts | 6 positions, 2 or 4 used | 8 positions, all 8 used |
| Width | ~9.65 mm | ~11.68 mm |
| Used for | Landline telephone, DSL, fax, modem | Ethernet, PoE, some serial console cables |
| Cable | Flat 2- or 4-conductor | 4 twisted pairs, Cat5e and above |
| Wiring standard | USOC | TIA/EIA-568A or 568B |
| Typical voltage | ~48 V DC idle, up to 90 V AC ringing | Signal only, or up to 57 V DC with PoE |
| Data rate | Up to ~24 Mbps on VDSL2 | 10 Mbps to 40 Gbps depending on category |
| Max distance | Kilometres (DSL, at reducing speed) | 100 m |
The Naming Is Technically Wrong
Worth knowing, because it explains the confusion. “RJ” stands for Registered Jack, a US telephone-network scheme that specifies both the connector and the wiring. The real RJ45 specification described a single-line data jack with a programming resistor — not Ethernet at all.
What everyone calls an RJ45 is properly an 8P8C connector (8 positions, 8 contacts). RJ11 is properly 6P2C or 6P4C. The RJ names stuck through common usage and now appear on the packaging, so they are what you will search and buy — but if you are reading a standards document, that is why the terminology does not match.
Pinouts
RJ45 — TIA/EIA-568B (the common one)
| Pin | Wire | Used for |
|---|---|---|
| 1 | White/Orange | Transmit + |
| 2 | Orange | Transmit − |
| 3 | White/Green | Receive + |
| 4 | Blue | Gigabit only |
| 5 | White/Blue | Gigabit only |
| 6 | Green | Receive − |
| 7 | White/Brown | Gigabit only |
| 8 | Brown | Gigabit only |
568A swaps the orange and green pairs. Either standard works — what matters is using the same one at both ends. A cable with 568A on one end and 568B on the other is a crossover cable, which modern equipment tolerates thanks to Auto-MDIX but which will confuse anyone who tests it later.
Note that 10/100 Ethernet uses only pins 1, 2, 3 and 6. Gigabit uses all four pairs bidirectionally. This is why a damaged cable can pass 100 Mbps traffic perfectly while failing to negotiate gigabit — a genuinely common fault.
RJ11
Wired to USOC, working outward from the centre:
- Pins 3 and 4 (the centre pair) — line 1, tip and ring
- Pins 2 and 5 — line 2, on a 6P4C connector
- Pins 1 and 6 — rarely used
Because RJ11 wires outward from the centre and RJ45 wires straight across, the two pinouts are structurally incompatible even where the pin counts overlap.
What Happens If You Mix Them
RJ11 plug into an RJ45 socket
It fits — the plug is narrower, so it slides in and sits off-centre. Two problems follow:
- The contacts do not align, so nothing works.
- Worse, the narrower plug can bend or displace the outer contacts in the RJ45 jack. Once bent, pins 7 and 8 may no longer make reliable contact, and the port loses gigabit capability permanently. This damages switch ports and laptop sockets and is not always obvious afterwards.
RJ45 plug into an RJ11 socket
It will not fit. The plug is too wide. Forcing it breaks the socket.
Telephone line into a network device
This is the one with real risk. A landline carries around 48 V DC at rest and rings at up to 90 V AC. Ethernet equipment is not designed for that. Connecting an active phone line to a switch port or NIC can destroy the port, and occasionally the device. Modern equipment often has surge protection, but it is not designed as a defence against being deliberately connected to a phone line.
Where They Meet in Practice
- DSL modems have both: RJ11 to the phone line, RJ45 to your network. The modem converts between them.
- Structured cabling often runs Cat5e/6 to every outlet and terminates some as telephone jacks. This works — an 8-wire cable can carry a phone line on one pair. Label them clearly, because the physical cable is identical.
- Cisco console cables use an 8P8C connector carrying RS-232, not Ethernet. Plugging a console cable into a network port does nothing; plugging a network cable into a console port also does nothing. Both are common mistakes.
- IP phones use RJ45 for both network and handset connections, having abandoned RJ11 entirely.
Identifying Them Quickly
- Count the gold contacts. Four (or two) means RJ11. Eight means RJ45. This is the fastest reliable check.
- Compare widths. RJ45 is noticeably wider, roughly the width of your little fingernail.
- Look at the cable. Flat and thin, usually grey or white, means telephone. Round with four twisted pairs means Ethernet.
- Check the socket label. Wall plates and devices are usually marked with a telephone or network icon.
Frequently Asked Questions
Can I plug an RJ11 cable into an RJ45 port?
Physically yes, functionally no — and it risks bending the outer contacts in the jack, which can permanently cost you gigabit capability on that port. Do not do it.
Can I use Ethernet cable for a telephone line?
Yes. Cat5e or Cat6 carries a phone line easily on one pair, and running Ethernet everywhere and terminating some outlets as phone jacks is standard practice in structured cabling. You need an RJ11 jack or plug at the ends.
Is RJ45 the correct name?
Not technically. The connector is an 8P8C; the original RJ45 registered-jack specification described something different. Everyone calls it RJ45, including manufacturers, so the name is practical even though it is inaccurate.
Why does my cable work at 100 Mbps but not gigabit?
10/100 uses only pins 1, 2, 3 and 6. Gigabit needs all eight. A break, a bad termination, or a bent jack contact on pins 4, 5, 7 or 8 lets 100 Mbps work while gigabit fails to negotiate. This is one of the most common cable faults there is.
Will plugging a phone line into a network card damage it?
It can. Ringing voltage reaches roughly 90 V AC, well beyond what Ethernet circuitry expects. Many devices survive it, but it is a real way to destroy a port.
What connector do I need for a Cisco console cable?
An 8P8C — the same physical connector as Ethernet — but carrying RS-232 serial. It is not electrically compatible with an Ethernet port despite fitting into one. See RJ45 connectors for termination options.
