Pick by speed AND distance:
- Cat 5e: 1 Gb/s, end of life — avoid for new builds.
- Cat 6: 1 Gb/s over 100 m, 10 Gb/s only up to ~37 m.
- Cat 6A: 10 Gb/s over 100 m — the structured-cabling standard in data centers.
- Cat 7 / 7A: heavier shielding (S/FTP) but proprietary non-RJ45 connectors, rarely used.
- Cat 8: 25 and 40 Gb/s, but over 30 m maximum — reserved for server↔switch top-of-rack links.
Shielding matters as much as the category: U/UTP (unshielded) in clean environments, F/UTP or S/FTP where there is interference. Field rule: Cat 6A copper for anything ≤10 Gb/s over 100 m; beyond that (25/40/100 Gb/s or long distances) switch to fiber.