Ethernet Networking

Ethernet at the Data Link Layer
Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly referred to as hardware addressing or MAC addressing. Ethernet is also responsible for framing packets received from the Network layer and preparing them for transmission on the local network through the Ethernet contention media access method. There are four different types of Ethernet frames available:

- Ethernet_II
- IEEE 802.3
- IEEE 802.2
- SNAP

Ethernet Addressing

Here�s where we get into how Ethernet addressing works. It uses the Media Access Control (MAC) address burned into each and every Ethernet Network Interface Card (NIC). The MAC, or hardware address, is a 48-bit (6-byte) address written in a hexadecimal format. Figure as below shows the 48-bit MAC addresses and how the bits are divided.

The organizationally unique identifier (OUI) is assigned by the IEEE to an organization. It�s composed of 24 bits, or 3 bytes. The organization, in turn, assigns a globally administered address (24 bits, or 3 bytes) that is unique (supposedly, again�no guarantees) to each and every adapter they manufacture. Look closely at the figure. The high-order bit is the Individual/Group (I/G) bit. When it has a value of 0, we can assume that the address is the MAC address of a device and may well appear in the source portion of the MAC header. When it is a 1, we can assume that the address represents either a broadcast or multicast address in Ethernet, or a broadcast or functional address in TR and FDDI (who really knows about FDDI?). The next bit is the G/L bit (also known as U/L, where U means universal). When set to 0, this bit represents a globally administered address (as by the IEEE). When the bit is a 1, it represents a locally governed and administered address (as in DECnet). The low-order 24 bits of an Ethernet address represent a locally administered or manufacturer-assigned code. This portion commonly starts with 24 0s for the first card made and continues in order until there are 24 1s for the last (16,777,216th) card made. You�ll find that many manufacturers use these same six hex digits as the last six characters of their serial number on the same card.

Ethernet Frames
The Data Link layer is responsible for combining bits into bytes and bytes into frames. Frames are used at the Data Link layer to encapsulate packets handed down from the Network layer for transmission on a type of media access. There are three types of media access methods: contention (Ethernet), token passing (Token Ring and FDDI), and polling (IBM mainframes and 100VG-AnyLAN).
The function of Ethernet stations is to pass data frames between each other using a group of bits known as a MAC frame format. This provides error detection from a cyclic redundancy check (CRC). But remember�this is error detection, not error correction. The 802.3 frames and Ethernet frame are shown in Figure as below.

Following are the details of the different fields in the 802.3 and Ethernet frame types:
Preamble An alternating 1,0 pattern provides a 5MHz clock at the start of each packet, which allows the receiving devices to lock the incoming bit stream.
Start Frame Delimiter (SFD)/Synch The preamble is seven octets and the SFD is one octet (synch). The SFD is 10101011, where the last pair of 1s allows the receiver to come into the alternating 1,0 pattern somewhere in the middle and still sync up and detect the beginning of the data.
Destination Address (DA) This transmits a 48-bit value using the least significant bit (LSB) first. The DA is used by receiving stations to determine whether an incoming packet is addressed to a particular node. The destination address can be an individual address, or a broadcast or multicast MAC address. Remember that a broadcast is all 1s (or Fs in hex) and is sent to all devices, but a multicast is sent only to a similar subset of nodes on a network.
Source Address (SA) The SA is a 48-bit MAC address used to identify the transmitting device, and it uses the LSB first. Broadcast and multicast address formats are illegal within the SA field.
Length or Type 802.3 uses a Length field, but the Ethernet frame uses a Type field to identify the Network layer protocol. 802.3 cannot identify the upper-layer protocol and must be used with a proprietary LAN�IPX, for example.
Data This is a packet sent down to the Data Link layer from the Network layer. The size can vary from 64 to 1500 bytes.
Frame Check Sequence (FCS) FCS is a field at the end of the frame that�s used to store the CRC. Let�s pause here for a minute and take a look at some frames caught on our trusty Etherpeek network analyzer. You can see that the frame below has only three fields: Destination, Source, and Type (shown as Protocol Type on this analyzer):

Destination: 00:60:f5:00:1f:27
Source: 00:60:f5:00:1f:2c
Protocol Type: 08-00 IP

This is an Ethernet_II frame. Notice that the type field is IP, or 08-00 in hexadecimal.
The next frame has the same fields, so it must be an Ethernet_II frame too:

Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 02:07:01:22:de:a4
Protocol Type: 81-37 NetWare

Included this one so you could see that the frame can carry more than just IP�it can also carry IPX, or 81-37h. Did you notice that this frame was a broadcast? You can tell because the destination hardware address is all 1s in binary, or all Fs in hexadecimal.
Now, pay special attention to the length field in the next frame; this must be an 802.3 frame:

Flags: 0x80 802.3
Status: 0x00
Packet Length: 64
Timestamp: 12:45:45.192000 06/26/1998
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 08:00:11:07:57:28
Length: 34

The problem with this frame is this: How do you know which protocol this packet is going to be handed to at the Network layer? It doesn�t specify in the frame, so it must be IPX. Why? Because when Novell created the 802.3 frame type (before the IEEE did and called it 802.3 Raw), Novell was pretty much the only LAN server out there. So, Novell assumed that if you were running a LAN, it must be IPX, and they didn�t include any Network layer protocol field information in the 802.3 frame.

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