Ethernet is the internal network over which PCs are connected (typically to a "Server" PC and to a cable modem or DSL modem for Internet access) over a Local Area Network (LAN) in the office or home. Ethernet is defined by the IEEE as the 802.3 standard. Almost all PCs and Macs come with Ethernet port... Ethernet can be added to older machines by plugging in a network adapter via a PCI Card or USB.