TL;DR — An RJ45 jack is a fixed copper 8P8C socket; an SFP port is an empty cage for a pluggable optic or DAC. Copper jack for short/cheap/PoE links, SFP for long/fast/fiber uplinks. Most boards use both.
RJ45 jack (the copper socket / 母座)
- Medium fixed at design time: twisted-pair copper, up to 100 m (TIA-568).
- VOOHU range: 10/100M, 1G, 2.5G, 5G, 10GBASE-T.
- Carries PoE (802.3af/at/bt) — contact current to 1.5 A. Only the copper jack can power the far end.
- Cheap, dense, DIP or SMT, magjack or discrete, shielded options.
SFP port (the pluggable cage)
- Not a transceiver — an SFF-MSA cage + connector for a hot-pluggable module.
- VOOHU range: SFP/SFP+/SFP28/QSFP...QSFP112, 5G to 400G.
- Fiber reach: ~300 m MMF to 10-80 km SMF; or short copper via DAC.
- No power over the link. Optics run hot — vent/heat-sink the cage.
Decide fast
- Needs PoE? -> RJ45 jack.
- Over 100 m or cross-building? -> SFP fiber.
- Up to 10G and up to 100 m? -> RJ45 jack (cheaper).
- 25G+ or medium flexibility? -> SFP.
Gotcha: no PoE over fiber; don't buy an SFP + optic for a short 10G copper hop.
Full write-up with spec tables (VOOHU makes both the jack and the cage): https://voohuelectronic-ux.github.io/voohu-rj45-guide/resources/rj45-jack-vs-sfp-copper-vs-fiber.html
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