Telephone Jack vs Ethernet Jack: RJ11, RJ45 and What Fits

An RJ11 telephone plug slides into an RJ45 Ethernet jack. That is the problem. The smaller plug lands on the two centre pins, makes contact, and appears to work, which convinces people that the two are interchangeable. They are not. The jack is wider, the pin count differs, the wiring standard differs, and the cable behind the wall may be Cat3 telephone wire that cannot support a modern Ethernet link. Understanding the physical and electrical differences prevents a slow, intermittent or damaging installation.

Physical Differences

RJ11 uses a 6-position modular housing, about 9.5 mm wide, and is usually fitted with 2 or 4 contacts, described as 6P2C or 6P4C. RJ45 uses an 8-position housing, about 11.7 mm wide, with 8 contacts, described as 8P8C. The latch and the contact spacing are similar enough that an RJ11 plug enters an RJ45 jack, but it sits in the centre and leaves the outer contacts unused. The reverse does not work: an RJ45 plug is too wide for an RJ11 jack.

Width and Pin Count

Six positions against eight, and two or four contacts against eight. The centre pins of RJ11 align with pins 4 and 5 of RJ45, which is why a phone line can appear on an Ethernet pair. The outer pins, which carry the other Ethernet pairs, are untouched.

What Fits in What

RJ11 fits RJ45, with poor retention and only partial contact. RJ45 does not fit RJ11. A RJ45 plug forced into a RJ11 jack can bend the outer pins and crack the housing. Adapters exist but do not change the cable behind the wall.

Wiring and Standards

Ethernet uses four twisted pairs on pins 1, 2, 3, 6, 4, 5, 7 and 8. 10/100BASE-T uses two pairs on pins 1, 2, 3 and 6, while Gigabit and faster use all four. Telephone wiring uses one pair on the centre pins, sometimes a second pair for a second line. The T568A and T568B colour codes define the Ethernet pinout, and mixing them at the two ends creates a crossover cable. Telephone wiring has no such standard beyond pair polarity, which is not critical for a plain phone.

Pinout Basics

In T568B, pin 1 is white-orange, pin 2 orange, pin 3 white-green, pin 4 blue, pin 5 white-blue, pin 6 green, pin 7 white-brown and pin 8 brown. A phone line on the centre pair uses pins 4 and 5, which are blue and white-blue in the same cable.

Two Pairs or Four

10/100 Ethernet needs two pairs and can sometimes run on older cable if the pairs are twisted. Gigabit needs four pairs and Cat5e or better. A telephone cable with two pairs cannot support Gigabit no matter how well it is terminated.

Can Telephone Cable Carry Ethernet?

Sometimes, over short distances, at low speed. Cat3 telephone cable has more untwisted length and different impedance, so it may support 10/100 over 10 to 20 m in a quiet environment. It will not support Gigabit reliably, and it will not support Power over Ethernet safely, because the conductor gauge and the pair count are inadequate. Cat5e installed for telephone service is a different case: if all four pairs are present and the cable is continuous, it can often be re-terminated for Ethernet.

Cat3 and DSL

Cat3 was designed for voice and low-speed data. DSL uses it beyond voice frequencies and works because the modem tolerates the cable. That tolerance does not extend to a 1000BASE-T link, which expects a controlled impedance and four pairs.

PoE and Current

PoE can deliver up to 60 W or more over two or four pairs. Thin telephone conductors and poor connectors heat up and drop voltage. Never assume a phone cable is safe for PoE because it fits the jack.

Converting, Testing and Safety

To reuse telephone wiring for Ethernet, identify the cable category, confirm that four pairs are present and continuous, and re-terminate both ends on RJ45 keystones or plugs following one wiring standard. Test with a cable certifier or at least a continuity and pair-map tester. If the cable is Cat3 or has only two pairs, run new Cat5e or Cat6. Keep telephone and Ethernet runs separated where possible, and label the jacks so the next person does not plug a phone into a PoE port.

Re-Termination Steps

Cut the old jack off, strip the jacket, separate the pairs, punch down on a keystone following T568B, and repeat at the other end. Test the map, then test a link. If the pairs are not twisted in the right pattern, the link may negotiate at 100 Mbps instead of 1 Gbps.

Safe Practice

Disconnect the line before working on it. Telephone lines carry a DC voltage and can ring at higher voltage. Do not plug a phone into a port that may carry PoE, and do not plug a laptop into a port wired for analog service.

RJ11 and RJ45 compared

Real-World Application Scenarios

A home office finds a phone jack on the wall and a desktop that needs wired internet. If the cable is Cat5e with four pairs, re-terminating both ends on RJ45 keystones gives a working Gigabit link. If it is two-pair Cat3, the link may negotiate at 100 Mbps over a short run but will be unreliable, and a new cable is the correct fix. A small business with a PoE access point must never reuse a telephone drop, because the current and pair count are outside the cable rating.

Related Guides on This Site

For telephone cable detail, read the RJ11 telephone cable guide and the telephone wiring standards guide. Ethernet cable selection is covered in the cat6 vs cat6a differences guide and the cat6 UTP LAN cable guide.

Frequently Asked Questions (FAQ)