Ethernet

Even after determining a destination with an IP address and deciding the next hop with a routing table, the problem of actually delivering data between devices on the same physical network remains. IP is a logical address, and when two servers plugged into the same switch need to communicate, a separate way to identify 'who is who physically' is needed. Without rules for actually carrying data across this segment, an IP packet knows its destination but cannot cross that last mile. Ethernet is the standard responsible for device identification and data delivery on this local network segment.

In the 1970s, when computers in the same building needed to exchange data, each vendor used proprietary protocols. Different companies' equipment could not communicate, and compatibility issues repeated every time a network was expanded or equipment was replaced. Ethernet became important not simply because it defined a cable specification, but because it standardized frame format and addressing so that any vendor's equipment could communicate on the same LAN. Initially, it was a bus structure sharing coaxial cable, but as traffic grew, collisions became frequent and performance degraded sharply. The transition to switch-based architecture allowed each port to communicate independently, and speeds steadily climbed. The fact that virtually all wired LANs today -- inside data centers, cloud virtual networks, and home routers -- run on Ethernet is the result of this standardization and evolution.

Ethernet uses a 48-bit MAC address uniquely assigned to each network interface card (NIC). To send data, the destination MAC address, source MAC address, the actual data (payload), and an error-detection FCS are bundled into an Ethernet frame for transmission. When the destination MAC address on the same network is unknown, a separate protocol called ARP (Address Resolution Protocol) translates the IP address to a MAC address. Switches learn which MAC address is connected to which port and build a MAC address table. When a frame arrives, the switch looks up the destination MAC in this table and sends it only to the matching port, preventing unnecessary traffic from spreading across the entire network. Early Ethernet used CSMA/CD on a shared-medium hub, but modern Ethernet uses switch-based full-duplex communication where collisions have been eliminated entirely. Speeds have evolved from 10 Mbps to standards up to 400 Gbps today, while the frame structure and addressing scheme remain the same.

Ethernet and VLANs both operate in the LAN environment but serve different roles. Ethernet is the base set of rules for delivering frames between devices on the same physical network, while VLANs logically separate a single physical switch into multiple independent networks. Without Ethernet, frames cannot be delivered at all; without VLANs, all devices on the same switch share one broadcast domain. Ethernet and Wi-Fi are also both LAN communication standards, but Ethernet uses wired cables while Wi-Fi uses wireless radio. Wired Ethernet provides stable latency and bandwidth with minimal interference, making it suitable for data center and server-to-server communication, while Wi-Fi is essential for mobility. The physical medium differs, but from the upper layers' perspective, both carry the same IP packets.

Ethernet is the foundation for virtually every scenario where devices exchange data directly within the same network: inter-server communication in data centers, office LANs, and virtual networks inside cloud VPCs. Even when you design subnets and assign IP addresses, the actual frame delivery is handled by Ethernet, so physical layer issues like switch port configuration or cabling problems also break IP-level connectivity. High-speed Ethernet (10G, 25G, 100G) is essential for storage networks, cluster inter-node communication, and backbone connections. Conversely, on WAN segments or internet segments where physical distance requires passing through routers, Ethernet frames are not delivered as-is but are reconstructed into new frames at the router. Ethernet is the delivery foundation for local networks, but it does not extend to determining paths between networks.