RJ45 / 8P8C Ethernet: T568A and T568B

Applies to: Contact assignments for T568A/B. This is not a rear punch-down layout or a PoE injection circuit.

When to use it

Making a straight-through patch lead or terminating a compatible data jack.

Why it works this way

Keeping each twisted pair together matters even when a basic continuity test passes.

Connection reference

Make the connection

1. Identify contact 1 using the connector drawing; plug and socket views can appear mirrored.
2. Use A at both ends or B at both ends for a straight-through lead. Preserve pairs 1-2, 3-6, 4-5 and 7-8.
3. Use a suitable cable tester; check pair assignment as well as opens and shorts.

Check before powering

• Rear IDC terminal order varies by jack; do not copy the plug's left-to-right order onto it.
• An 8P8C socket may carry non-Ethernet signals. Confirm the equipment documentation before connecting it.

Sources & review

Source documents inspected; independent electrical and bench review pending. Reviewed 2026-09-27.

• Leviton Cat 5e / Cat 6 jack instructions
  Table 1: T568A/B contact assignments and jack termination labels · checked 2026-09-27
• Fluke Networks: RJ connector naming
  RJ11 / RJ14 / RJ25 versus modular connector contact counts · checked 2026-09-27

Original writing and diagram by Kal / Valleytech Solutions, CC BY 4.0. Manufacturer artwork and documentation retain their own rights. CC BY 4.0

Continue learning

• Phone jacks: RJ11, RJ14 and USOC terminals
• RS-485: a half-duplex multidrop bus

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