If an Ethernet link is up on my end, does it necessarily mean the computer on the other end is alive and I can talk to him?

Or does it mean that my PHY is just ready to send data onto the medium?

I know there are various ways to check "link state" and that there are operational vs administrative states of an interface.

  • When a Windows machine is powered down the link is still up and ready to receive a magic packet Wake on LAN. The indication can be various but normally the state is link and protocol up at 10 Mbit full duplex.
    – user36472
    Jun 28, 2017 at 11:06
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    – Ron Maupin
    Aug 7, 2017 at 16:28

1 Answer 1


Usually the link goes up on both sides at the same time. In very rare cases (damaged cable or port) it may only come up on one side.

Whether you can actually "talk" to the other computer depends on what protocols are running. TCP/IP requires compatible static addresses or DHCP and so on.

Cown's reference to Wake on LAN is valid - a sleeping WoL-capable node has the link up but ignores everything but a magic packet.

  • Thanks. I definitely agree with the 2nd statement. The first statement is the part I have been trying to track down and struggle with. Does the link go up on both sides due to auto negotiation? If so, what if it is disabled?
    – user_ABCD
    Jun 28, 2017 at 17:01
  • I think link pulses are stll required even when auto negotiation is (completely) deactivated. Additionally, the PHY needs to detect the carrier which is continuous for full-duplex operation. [reading Clause 28.1.4 now...]
    – Zac67
    Jun 28, 2017 at 18:06
  • No - link pulses are only used for 10BASE-T. 100BASE-TX w/o autoneg relies on the carrier/pause symbols.
    – Zac67
    Jun 28, 2017 at 18:12
  • Obviously, this is one of the situations when only one side might be lighting up, ie. when one pair is dead.
    – Zac67
    Jun 28, 2017 at 18:17

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