I want to connect 1000Base-T Ethernet to 100Base-TX Ethernet. 1000Base-T MDI lines have 8 signals whereas 100Base-TX has 4 signals. Does this create any problem if I connect both of them or auto-negotiation works such a way that only 4 signals will be used from 1000Base-T? Thanks

  • use a router...
    – Math Keeps Me Busy
    Mar 10, 2022 at 13:43
  • 2
    @MathKeepsMeBusy Seriously, you don't need a router for that. Maybe you don't mean a router?
    – Justme
    Mar 10, 2022 at 13:46
  • @Justme, Fair point. Router seems to imply NAT these days, but it didn't used to. A switch then. Not to advertise, but something premade like tigerdirect.com/applications/searchtools/… will probably end up cheaper than designing and building one's own 2 port interface.
    – Math Keeps Me Busy
    Mar 10, 2022 at 14:00
  • 2
    @MathKeepsMeBusy No, a router does not imply NAT, a router actually routes IP packets over a network while a switch just switches Ethernet packets between LAN devices. And you don't need a switch either, gigabit devices can talk directly with 100M devices.
    – Justme
    Mar 10, 2022 at 15:02

1 Answer 1


All consumer hardware you encounter that can do 1000Base-T (Gigabit Ethernet) can also do Fast Ethernet (100Base-TX) and will downgrade as necessary. Auto-Negotiation just works.

Doesn't apply to all hardware, though. I know of a class of sampling devices whose ethernet frames need Gigabit Ethernet, so they simply don't work with 100Base.


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