Network interfaces and addresses in Linux

Network interface is a generic term for physical and virtual network links that can carry addresses. The correspondence between physical network cards and network interfaces as the kernel sees them is not one-to-one. A network card with four ports is a single PCI device, but every one of its ports is seen as a separate link by the kernel.

There are also virtual links. Some virtual links are tied to physical network ports, but other types are fully independent. For example, MACVLAN links allow administrators to send traffic from certain IP addresses using a different MAC address. Since an Ethernet interface by definition cannot have multiple MAC addresses, Linux solves that problem by creating virtual interfaces on top of a physical Ethernet port and assigning different MAC and IP addresses to it. Multiplexing Ethernet traffic using 802.1Q VLAN or 802.3ad QinQ (nested VLAN) is also done by creating a virtual link that is bound to a specific...