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The original Ethernet standards from the 1970s were designed for nodes all connected to a shared electrical bus that often took the form of a thick yellow cable (you may have heard the term Thicknet). Whenever one node would transmit a signal, all other nodes connected to the cable would receive it. Communication was half-duplex, and all nodes were in the same collision domain. As a way of detecting errors introduced by collisions, the original Ethernet II (DIX) specification got a frame check sequence (FCS, sometimes called a cyclic redundancy check, or CRC) to detect errors. Even today as back then, nodes discard frames that fail the FCS check.

The multi-access nature of Ethernet made it necessary to assign each node's network interface a unique, 48-bit Media Access Control (MAC) address. The sending node would construct a frame that included the destination node's MAC address and the data to send. All nodes would receive the frame, but only the destination node would process it.

Now let's fast-forward to today. We still use MAC addresses, even though the original rationale for using them is long gone. To maintain backward compatibility over the years, we never got rid of them. Figure 1.3 shows the original DIX frame format that we still use today. We're still using a technology designed specifically for devices that were all sharing a thick yellow cable. Today, however, instead of nodes sharing this thick yellow cable, they're connected to a switch.

Figure 1.3 Layer 2 frame and layer 1 packet, structurally identical to the revised (1997) IEEE 802.3 format that we use today

You may have seen diagrams that show the Ethernet frame with an 8-byte preamble at the beginning. The preamble is not actually part of the frame but is a series of bits that provide clock synchronization for the Physical layer and signal the start of the frame. The entire collection of bits—including the preamble and frame—compose a layer 1 Ethernet packet. Although most of the time when you hear “packet” it refers to an IP packet (layer 3), “packet” is a generic term for any PDU. To avoid confusion, you can think of the raw bits as a layer 1 Ethernet PDU.

Switches replace the shared cable of the early Ethernet with multiple cables, breaking the inherent broadcast nature of the thick yellow cable. Switches thus have to perform some interesting hackery to maintain backward compatibility with the early Ethernet standards. When a switch receives an Ethernet frame, by default it forwards that frame to all other devices connected to the switch—a process called flooding or broadcasting. This creates the illusion that all nodes are connected to the same thick yellow cable. (In networking parlance, they're all in the same broadcast domain or segment.) This illusion is called transparent bridging (aka switching) because to the nodes, the switch is invisible. Incidentally, you'll recognize this simulated yellow cable by its common name: a local area network (LAN).