2.2 Interpret Ethernet Frame Format

Ethernet addressing uses the "Media Access Control"(MAC) Address burned into each network interface card. This is a 48 bit hexadecimal address.

The first 24 bits or 3 bytes are the OIU, organizationally unique identifier, which is assigned to each adapter manufacturer by the IEEE.

• If the first bit I/G(Individual/Group) is a 0, we can assume this is the MAC address of a device. If this is a 1 we can assume that address represents either a broadcast or multicast address.
• The next bit G/L or U/L bit. (Global or Universal/Local)- If set to 0 this means it was assigned by the IEEE. If set to 1 this it was locally addressed.

The data link layer is responsible for encapsulating a packet and turning it into a Data Frame-

• Preamble- An alternating 1,0 pattern, provides a 5MHz clock at the start of each packet to lock the incoming bit stream
• Start Fram Delimiter(SFD)/synch- Part of the preamble above- The last byte- I don't get this one.
• Destination Address- a 48 bit value using the Least Significant Bit (LSB) first. Used by receiving stations to determine if the packet is addressed to a specific node. These can also be broadcast address(all 1's in binary, all f's in Hex) or multicast.
• Source Address- 48 Bit MAC address from the sending device. Broadcast and Multicast addresses are illegal in this field.
• Type or Length- 802.3- Specifies the protocol(IP, IPv6).
• Data- This is a packet sent down from the Data Link Layer- Size can vary from 46-1500 Bytes.
• Frame Check Sequence(FCS)- Used to store a Cyclical Redundancy Check(CRC) answer. A mathematical algorithm that is run when each frame is built. Hosts receive the frame and check this number. If the answer is not the same the frame is discarded as errors are assumed.