Duplex describes whether a network link can send and receive data at the same time. Duplex mismatches were a common cause of slow or unreliable Ethernet connections on older networks.
Half duplex
Half duplex means a link can send or receive at a given moment, but not both at the same time. This was common with older hubs and shared Ethernet segments.
Full duplex
Full duplex means the link can send and receive at the same time. Modern switched Ethernet networks normally use full duplex with auto-negotiation.
Check link settings
sudo ethtool eth0Replace eth0 with the actual interface name. Use ip link to list interfaces.
Modern note
Most modern networks should use auto-negotiation. Forcing duplex or speed settings is usually only needed when troubleshooting specific older hardware or managed switch ports.