Ethernet and ARP

Network interface

Also called network card, NIC, eth0, link state, ip link, carrier, LOWER_UP, NO-CARRIER

A card a machine sends and receives through, switched on or off, with or without a live link.

ip link lists them: eth0, wlan0, lo for the machine itself. Two flags matter first: UP, the card is switched on (ip link set eth0 up), and LOWER_UP, its link is live: a cable into a working switch port, or a Wi-Fi association. UP without a link shows NO-CARRIER. A card that is down sends nothing, and the kernel drops every route through it; its addresses stay, and come back to life with it.

See it on real machines

Learn it on real machines.