2.1 Legacy Media Types
2.1.7 LAN frames and hex values
When stations transmit to each other on a LAN, they format the data in a structured manner so that devices know what octets signify what information. Various frame formats are available. When you configure a device, you must define the format that your station will use, realizing that more than one format might be configured, as is the case for a router.

Figure illustrates four common frame formats for Ethernet. Some users interchange the terms packets and frames rather loosely. However, according to RFC 1122, a significant difference does exist. Frames refer to the entire message, from the data link layer (Layer 2) header information through and including the user data. Packets exclude Layer 2 headers and include only the Layer 3 protocol header through and including user data.

The frame formats developed as the LAN industry and associated protocol requirements evolved. When Xerox developed the original Ethernet (which was later adopted by the industry), a frame format like the Ethernet frame in Figure was defined. The first six octets contain the destination MAC address, and the next six octets contain the source MAC address. Two bytes following that indicate to the receiver the type of Layer 3 protocol encapsulated within the data portion of the frame. For example, if the frame encapsulates an IP packet, then the type field value is 0x0800. Figure lists several common protocols and their associated type values.

Following the type value, the receiver expects to see a protocol header. For example, if the type value indicates that the packet is IP, the receiver expects to decode IP headers next. If the value is 8137, the receiver decodes the encapsulated packet as a Novell packet.

IEEE defined an alternative frame format. In the IEEE 802.3 formats, the source and destination MAC addresses remain, but instead of a type field value, the packet length is indicated. Three derivatives of this format are used in the industry: raw 802.3, 802.3 with 802.2 Logical Link Control (LLC), and 802.3 with 802.2 and Subnetwork Access Protocol (SNAP).

A receiver recognizes that a packet follows 802.3 formats rather than Ethernet formats by the value of the 2-byte field following the source MAC address. If the value falls within the range of 0x0000 and 0x05DC (1500 decimal), the value indicates length; protocol type values begin after 0x05DC, in which case the frame type is Ethernet Version II. Further, if the 16-bit value following the type/length field is 0xAAAA, then the frame is a SNAP (or IEEE 802.3 SNAP) frame; if this value is 0xFFFF, then the frame is a raw 802.3 (or Novell 802.3 raw) frame; otherwise, it is a 802.3 with 802.2 LLC (or IEEE 802.3 ) frame.