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.
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 1 shows the 48-bit MAC addresses and how the bits are divided.
FIGURE 1 Ethernet addressing using MAC addresses
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 it manufactures. 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 global/local bit, or just 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 what DECnet used to do).
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.
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 2
Note:Encapsulating a frame within a different type of frame is called tunneling.
FIGURE 2 802.3 and Ethernet frame formats
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 1,500 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 OmniPeek 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 (mostly just referred to as 0x800) 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: 08-00 IP
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.
Let’s take a look at one more Ethernet_II frame. I’ll talk about this next example again when we use IPv6 in this blog, but you can see that the Ethernet frame is the same Ethernet_II frame we use with the IPv4 routed protocol but the type field has 0x86dd when we are carrying IPv6 data, and when we have IPv4 data, we use 0x0800 in the protocol field:
Destination: IPv6-Neighbor-Discovery_00:01:00:03 (33:33:00:01:00:03)
Source: Aopen_3e:7f:dd (00:01:80:3e:7f:dd)
Type: IPv6 (0x86dd)
This is the beauty of the Ethernet_II frame. Because of the protocol field, we can run any Network layer routed protocol and it will carry the data because it can identify the Network layer protocol.
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 1 shows the 48-bit MAC addresses and how the bits are divided.
FIGURE 1 Ethernet addressing using MAC addresses
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 it manufactures. 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 global/local bit, or just 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 what DECnet used to do).
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.
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 2
Note:Encapsulating a frame within a different type of frame is called tunneling.
FIGURE 2 802.3 and Ethernet frame formats
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 1,500 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 OmniPeek 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 (mostly just referred to as 0x800) 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: 08-00 IP
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.
Let’s take a look at one more Ethernet_II frame. I’ll talk about this next example again when we use IPv6 in this blog, but you can see that the Ethernet frame is the same Ethernet_II frame we use with the IPv4 routed protocol but the type field has 0x86dd when we are carrying IPv6 data, and when we have IPv4 data, we use 0x0800 in the protocol field:
Destination: IPv6-Neighbor-Discovery_00:01:00:03 (33:33:00:01:00:03)
Source: Aopen_3e:7f:dd (00:01:80:3e:7f:dd)
Type: IPv6 (0x86dd)
This is the beauty of the Ethernet_II frame. Because of the protocol field, we can run any Network layer routed protocol and it will carry the data because it can identify the Network layer protocol.
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