RJ Connector Pinouts — RJ11 to RJ45

The registered-jack designations decoded: what each RJ code actually is, the positions-and-contacts (6P2C, 6P4C, 8P8C) behind it, which pins carry which line under the USOC center-outward convention, and where the names get misused. RJ codes are a service-registration convention from the Bell era — the connector sizes are the facts that matter on the bench.

Registered jack designations

USOC
Each RJ designation with its connector size (positions/contacts), the lines it serves, and pin assignments. 'Pins used' follows the USOC convention — line 1 always on the center pair, additional lines nesting outward.
DesignationConnectorLinesPins usedNote
RJ116P2C1 lineLine 1 on center pins 3–4The colloquial name for nearly every 6-position phone plug
RJ146P4C2 linesLine 1 on 3–4, line 2 on 2–5What a two-line "RJ11" cord actually is
RJ256P6C3 linesLine 1 on 3–4, line 2 on 2–5, line 3 on 1–6All six positions populated
RJ126P6C1 line + key-system controlAll six contactsKey telephone system wiring, not a third line
RJ618P8C4 linesUSOC center-outward across 8 positionsPairs 3–4 sit too far apart for data — never use for Ethernet
"RJ45" (8P8C)8P8CData (4 pairs)T568A or T568BThe Ethernet connector — properly 8P8C; the true keyed RJ45S was a telephone-data jack
6P2C, 6P4C, and 6P6C plugs share the same shell — the difference is how many contacts are loaded. Nearly everything 6-position gets called “RJ11” in the field regardless of contacts; the designation only becomes precise when a second or third line is involved.

The center-outward rule

USOC assigns pairs from the middle of the connector toward the edges: line 1 on the center pins, line 2 wrapped around it, line 3 outside that. The payoff is backward compatibility — any single-line device plugs into any multi-line jack and finds line 1. The cost shows up in RJ61, the 8-position USOC layout: its outer pairs sit so far apart electrically that they are useless for high-speed data, which is why data jacks wire to T568A/B instead — a layout designed to keep each transmission pair on adjacent pins.

Common questions

What is the difference between RJ11 and RJ45?

Size and purpose. RJ11 is a 6-position telephone plug using the center two contacts (6P2C) for one phone line; "RJ45" is the 8-position, 8-contact (8P8C) data connector carrying four pairs for ethernet. An RJ11 plug physically clicks into an RJ45 jack — a habit worth avoiding, since the narrower plug can deform the jack's outer contacts.

Why is RJ45 technically the wrong name?

The registered-jack codes (RJ) describe a wiring/service registration from the Bell System USOC scheme, not a physical connector. The true RJ45(S) was a keyed telephone-data jack with one pair and a programming resistor — mechanically incompatible with an ethernet port. The ethernet connector is properly "8P8C," but the industry has called it RJ45 for forty years and no one is stopping. Knowing the distinction matters exactly once: when ordering keyed connectors from a telecom catalog.

What are RJ14 and RJ25?

The same 6-position plug as RJ11 with more contacts loaded: RJ14 (6P4C) carries two lines — line 1 on center pins 3/4, line 2 on pins 2/5 — and RJ25 (6P6C) carries three, adding line 3 on the outer pins 1/6. That center-outward nesting is the USOC pattern: a one-line plug works in a three-line jack because line 1 always sits in the middle.

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