Ethernet/802.3 Frames, Continued

Source and Destination Address

each 6 bytes (48 bits!), and are uniquely assigned by IEEE. This is called a station's MAC address.

• All stations on a segment examine the destination address of all frames to see if it was addressed to them. If the destination address matches its own address, it accpets the frame for processing.
• Mac addresses are in two parts: 3 bytes of manufacturer ID and 3 bytes of serial number.

 

Type field

indicates which higher-level protocol created this frame, eg 0x0800 for IP. In 802.3^[3] this field gives the length (in bytes) of the data field.
 

Data field

between 46 and 1500 bytes of data. NB: minimum frame size is thus 64 bytes. The data field usually contains an IP datagram.