Why does ethtool show 'no link' for ethernet interface even though cable is physically connected?

Issue

• The ethtool command shows Link detected: no for interface
• The ip link command shows UP but NO CARRIER
• The mii-tool command shows SIOCGMIIPHY on 'ethX' failed
• The network link light is on but the interface will not come up
• The message ADDRCONF(NETDEV_UP): ethX: link is not ready appears in kernel messages