One of the most significant strengths of the Ethernet family of protocols is that these protocols use the same data-link standard. In fact, the core parts of the data-link standard date back to the original Ethernet standards.
The Ethernet data-link protocol defines the Ethernet frame: an Ethernet header at the front, the encapsulated data in the middle, and an Ethernet trailer at the end. Ethernet actually defines a few alternate formats for the header, with the frame format shown in Figure 2-18 being commonly used today.
Figure 2-18 Commonly Used Ethernet Frame Format
While all the fields in the frame matter, some matter more to the topics discussed in this book. Table 2-6 lists the fields in the header and trailer and a brief description for reference, with the upcoming pages including more detail about a few of these fields.
Table 2-6 IEEE 802.3 Ethernet Header and Trailer Fields
| Field | Bytes | Description |
| Preamble | 7 | Synchronization. |
| Start Frame Delimiter (SFD) | 1 | Signifies that the next byte begins the Destination MAC Address field. |
| Destination MAC Address | 6 | Identifies the intended recipient of this frame. |
| Source MAC Address | 6 | Identifies the sender of this frame. |
| Type | 2 | Defines the type of protocol listed inside the frame; today, most likely identifies IP version 4 (IPv4) or IP version 6 (IPv6). |
| Data and Pad* | 46– 1500 | Holds data from a higher layer, typically an L3PDU (usually an IPv4 or IPv6 packet). The sender adds padding to meet the minimum length requirement for this field (46 bytes). |
| Frame Check Sequence (FCS) | 4 | Provides a method for the receiving NIC to determine whether the frame experienced transmission errors. |
* The IEEE 802.3 specification limits the data portion of the 802.3 frame to a minimum of 46 and a maximum of 1500 bytes. The term maximum transmission unit (MTU) defines the maximum Layer 3 packet that can be sent over a medium. Because the Layer 3 packet rests inside the data portion of an Ethernet frame, 1500 bytes is the largest IP MTU allowed over an Ethernet.
The source and destination Ethernet address fields play a huge role in how Ethernet LANs work. The general idea for each is relatively simple: The sending node puts its own address in the source address field and the intended Ethernet destination device’s address in the destination address field. The sender transmits the frame, expecting that the Ethernet LAN, as a whole, will deliver the frame to that correct destination.
Ethernet addresses, also called Media Access Control (MAC) addresses, are 6-byte-long (48-bit-long) binary numbers. For convenience, most computers list MAC addresses as 12-digit hexadecimal numbers. Cisco devices typically add some periods to the number for easier readability as well; for example, a Cisco switch might list a MAC address as 0000.0C12.3456.
Most MAC addresses represent a single NIC or other Ethernet port, so these addresses are often called a unicast Ethernet address. The term unicast is simply a formal way to refer to the fact that the address represents one interface to the Ethernet LAN. (This term also contrasts with two other types of Ethernet addresses, broadcast and multicast, which will be defined later in this section.)
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