CCNA Routing and Switching
<< IP and Network Fundamentals Course
>> Models, Protocols, and Standards Section
section table << IP and Network Fundamentals Course
>> Models, Protocols, and Standards Section
- Understanding Ethernet MAC Addresses
Media Access Control meant to to be a Physical Hardware Address System used by Ethernet. Understanding Ethernet MAC Addresses will help you grasp how Ethernet transfer frames filled with packets across network devices using MAC Addresses format. So, if Network Layer package Packets using IP Addressing system which is a logical Addressing system, then Ethernet frames or load those packets using a physical Addressing system that meant on the hardware level.
Each network device in this world get its own unique 48bit MAC Address which will be used only by Ethernet Technology. The moment you hear MAC address, think Ethernet is being used regardless of the Media (cable type) is being used between the devices such Copper, Fiber cables, or even over Wireless radio wave.
- On Ethernet NICs, the physical or MAC address (also called the hardware address) is expressed as 12 hexadecimal digits, arranged in pairs with colons between each pair, for example, 12:3A:4D:66:3A:1C. In binary notation, this translates to a 48-bit (or 6-byte) number, with the initial 3 bytes representing the manufacturer and the last 3 bytes.
- MAC Address is a unique 6-byte (48-bit) address that is usually permanently burned into a network interface card or other physical-layer networking device and that uniquely identifies the device on an Ethernet-based network. A MAC address is also known as an Ethernet address, hardware address, physical address, or PHY address.
Table of Contents
If the least significant bit of the first octet is set to 1 (i.e. The second hexadecimal digit is odd) the frame will still be sent only once; however, NICs will choose to accept it based on criteria other than the matching of a MAC address: for example, based on a configurable list of accepted multicast MAC addresses.
Ethernet MAC Address
All Ethernet addresses (Including Unicast, Broadcast, and Multicast) are 6-byte-long (48-bit-Long) or 12-character Hexadecimal Address. Most computers list this address as 12-digit Hexadecimal numbers. Cisco divides it to 3 groups: E.g. 2001.1111.1111
An Ethernet MAC address is a hardware address, therefore, it MUST uniquely identifies every Ethernet device in the world. When vendors create network devices such Ethernet NICs, wireless devices, routers, and switches, they burn-in these addresses into their devices.
A MAC address has few names, including Physical Address, Ethernet Address, Burned-in Address, and Hardware Address. No matter what it’s called, it is a 12-character hexadecimal Address. Examples:
- Cb54.5689.90cc (Cisco Way of Displaying MAC Address)
- 18:40:58:76:90:hh (Linux Way of Displaying MAC Address)
Types of MAC addresses
1. Unicast Address
Is another name for MAC address or an Ethernet Address. Every MAC address represent a single NIC in the world; hence, it is called Unicast (Unique) Global Address.
In addition to unicast addresses, Ethernet also uses group addresses. Group addresses identify more than one LAN interface card. A frame sent to a group of addresses might be delivered to a small set of devices on the LAN, or even to all devices (everyone) on the LAN. So the IEEE defines categories of group addresses for Ethernet communications:
2. Broadcast Address
Meaning to everyone – Frames sent using this address should be delivered, received, and processed by all devices on the Ethernet LAN or VLAN.
Broadcast conclude 12-digit Hexadecimal address with value of FFFF.FFFF.FFFF inserted into the destination filed of the Frame.
3. Multicast Address
To a specific group – Frames sent using Multicast Ethernet address will be copied and forwarded to a subset of devices on the LAN that volunteers to receive such frames which sent to a specific multicast address – for specific group only and NOT to everyone. Chrome download standalone 32 bit. It is a 12-digit Hexadecimal with most of the time carries the following value 0100.5E7F.XXXX, this is an official (IANA) Multicast address.
Note: Unicast Address is the mostly used Media Access Control Address among network devices.
Address Resolution Protocol
How would PC1 figure out R3’s Unicast MAC Address for the first time?
In order to learn R3 MAC address, PC1 announces or broadcast through Address Resolution Protocol (ARP). PC1 sends broadcast frame using ARP Message Request, creates an IP packet indicating R3’s IP Address (192.168.1.1) as destination (which is known to PC1 through DHCP or Static IP configuration), and encapsulates the packet inside a frame indicating an ARP Message Request as Broadcast address (FFFF.FFFF.FFFF) inside the frame.
After receiving an ARP Reply Message and learning R3’s MAC address, PC1 can now use R3 as next hop for outside routing, and can create an IP Packet destined to Web-server’s IP address. PC1 in the following figure creates a new IP packet indicating this time Web-Server’s IP address as destination IP address (172.16.10.20), and encapsulates the IP packet inside a Frame heading to R3’s MAC address as next-hop since PC1 knows now the R3 MAC address.
Multicast Address
Multicast frame used in special cases for a specific group of computers. The following figure shows a Multicast Frame message, e.g. from a Ghost-Server heading to group of computers on the network and PC1 could be one of them. Usually, devices volunteer through a Ghost Client loaded by CD or USB on the PC to receive this type of message.
MAC Addresses Distribution
So how would an Ethernet Address Assigned to Manufactures? Since it must be a unique number, all Ethernet devices are assigned a Universal Unique MAC Address. Before a manufacture can build Ethernet NICs or Interfaces, it must ask the IEEE group to assign the manufacture a Universally Unique 3-byte Code, called Organizational Unique Identifier (OUI).
Ethernet’s NIC or Interface Hardware Address
Media Access Control Address (MAC), Burned-in Address, data link address, hardware address, physical address, or MAC address, all these names refer to the same exact 12-hex digit hardware address that is used by any NIC or interface in the world (1001.2222.3333 – Cisco style MAC Address) and (AF:02:CD:EE:71:AB – PC style MAC Address). Remember, Network devices initially need the help of Address Resolution Protocol (ARP) to learn each other MAC addresses once they are plugged into an Ethernet LAN network switch or hub.
Is it IPv4 or IPv6 Frame?
Identifying which Network Layer Protocol being used is crucial for Network devices to distinguish when Dual Stack is being used (IPv4 and IPv6 Internet). The Ethernet Type Field (EtherType) sits in the Ethernet data link layer header, but its purpose is to directly help the network processing (TCP/IP Stack) on routers and hosts.
Basically, the Type Field identifies what type of network layer (Layer 3) is being used inside the packet that sits inside the Ethernet frame. Internet uses 2 Types of Internet Protocols: IPv4 and IPv6.
So, what number should the sender put in the header to identify an IPv4 Packet or an IPv6 packet? As it turns out, the IEEE manages a list of EtherType values, so that every network layer protocol needs a unique EtherType value can have a number, however, only 2 types are being used these days: IPv4 and IPv6.
- IPv4 value in Hexadecimal is: 0800
- IPv6 value in Hexadecimal is: 86DD
Notice that the Ethernet Frame doesn’t change to accommodate IPv6 packet. Ethernet keeps the same frame as is whether IPv4 or IPv6 is being used, the only thing changes is: the Ethernet Type Field number:Either 0800 for IPv4 or 86DD for IPv6. Chrome 32 bit download.
Layer 2 Error detection FCS
The Ethernet Frame Check Sequence (FCS) field in the Ethernet Trailer – the only field in the ethernet trailer – gives the receiving node a way to compare results with the sender, to discover whether errors occurred to the frame on the way.
Note that error detection does not also means error recovery, ethernet defines that the frame with error should be discarded, and ethernet does not attempt to recover the frame. It is the TCP protocol at Layer 4 (Transport Layer) where it recover the lost data by noticing that it is lost and sending the data again. (FCS IS NOT AN ERROR RECOVERY)
Recommended Readings
By Wikipedia MAC Address | IEEE.org | HighTech Ethernet Explained | Router Ally Ethernet Guide | Whatismyipaddress MAC Explained | Cisco CCNA Simplified | Cisco CCNA in 60 Days | Networking for Dummies | CCNA R&S Certification Kit | Ethernet Definitive Guide
'Understanding Ethernet MAC Addresses', 5 out of 5 based on 1 ratings.CCNA Routing and Switching
<< IP and Network Fundamentals Course
>> Models, Protocols, and Standards Section
section table << IP and Network Fundamentals Course
>> Models, Protocols, and Standards Section
- Understanding Ethernet MAC Addresses
ALSO ON CCNA HUB
Ethernet MAC Address (7.2)
Ethernet technology relies on MAC addresses to function. MAC addresses are used to identify the frame source and destination.
MAC Address and Hexadecimal (7.2.1)
As discussed in detail in Chapter 5, “Number Systems,” in networking, IPv4 addresses are represented using the decimal (base 10) number system and the binary (base 2) number system. IPv6 addresses and Ethernet addresses are represented using the hexadecimal (base 16) number system. To understand hexadecimal, you must first be very familiar with binary and decimal.
The hexadecimal numbering system uses the numbers 0 to 9 and the letters A to F.
An Ethernet MAC address consists of a 48-bit binary value. Hexadecimal is used to identify an Ethernet address because a single hexadecimal digit represents 4 binary bits. Therefore, a 48-bit Ethernet MAC address can be expressed using only 12 hexadecimal values.
Figure 7-5 compares the equivalent decimal and hexadecimal values for binary 0000 to 1111.
Figure 7-5 Decimal to Binary to Hexadecimal Conversion
Given that 8 bits (1 byte) is a common binary grouping, binary 00000000 to 11111111 can be represented in hexadecimal as the range 00 to FF, as shown in the Figure 7-6.
Figure 7-6 Selected Examples of Decimal to Binary to Hexadecimal Conversions
When using hexadecimal, leading zeros are always displayed to complete the 8-bit representation. For example, in Figure 7-6, the binary value 0000 1010 is shown to be 0A in hexadecimal.
Hexadecimal numbers are often represented by a value preceded by 0x (for example, 0x73) to distinguish between decimal and hexadecimal values in documentation.
Hexadecimal may also be represented using a subscript 16 or by using the hex number followed by an H (for example, 73H).
You might have to convert between decimal and hexadecimal values. If such conversions are required, convert the decimal or hexadecimal value to binary and then to convert the binary value to either decimal or hexadecimal as appropriate. See Chapter 5 for more information.
Ethernet MAC Address (7.2.2)
In an Ethernet LAN, every network device is connected to the same shared medium. The MAC address is used to identify the physical source and destination devices (NICs) on the local network segment. MAC addressing provides a method for device identification at the data link layer of the OSI model.
An Ethernet MAC address is a 48-bit address expressed using 12 hexadecimal digits, as shown in Figure 7-7. Because 1 byte equals 8 bits, we can also say that a MAC address is 6 bytes in length.
Figure 7-7 Ethernet MAC Address in Bits, Hextets, and Bytes
All MAC addresses must be unique to the Ethernet device or Ethernet interface. To ensure uniqueness, every vendor that sells Ethernet devices must register with the IEEE to obtain a unique 6-digit hexadecimal (that is, 24-bit or 3-byte) code called an organizationally unique identifier (OUI).
When a vendor assigns a MAC address to a device or to an Ethernet interface, the vendor must do as follows:
- Use its assigned OUI as the first 6 hexadecimal digits.
- Assign a unique value in the last 6 hexadecimal digits.
Therefore, an Ethernet MAC address consists of a 6-digit hexadecimal vendor OUI code followed by a 6-digit hexadecimal vendor-assigned value, as shown in Figure 7-8.
For example, say that Cisco needs to assign a unique MAC address to a new device, and the IEEE has assigned Cisco the OUI 00-60-2F. Cisco would configure the device with a unique vendor code such as 3A-07-BC. Therefore, the Ethernet MAC address of that device would be 00-60-2F-3A-07-BC.
It is the responsibility of a vendor to ensure that no two of its devices are assigned the same MAC address. However, it is possible for duplicate MAC addresses to exist because of mistakes made during manufacturing, mistakes made in some virtual machine implementation methods, or modifications made using one of several software tools. In such a case, it is necessary to modify the MAC address with a new NIC or make modifications by using software.
Frame Processing (7.2.3)
Sometimes a MAC address is referred to as a burned-in address (BIA) because the address is hard coded into read-only memory (ROM) on the NIC. This means that the address is permanently encoded into the ROM chip.
When the computer boots up, the NIC copies its MAC address from ROM into RAM. When a device is forwarding a message to an Ethernet network, as shown in Figure 7-9, the Ethernet header includes the following:
- Source MAC address: This is the MAC address of the source device NIC.
- Destination MAC address: This is the MAC address of the destination device NIC.
Figure 7-9 The Source Prepares a Frame to Send to the Destination
When a NIC receives an Ethernet frame, it examines the destination MAC address to see if it matches the physical MAC address that is stored in RAM. If there is no match, the device discards the frame. In Figure 7-10, H2 and H4 discard the frame. The MAC address matches for H4, so H4 passes the frame up the OSI layers, where the de-encapsulation process takes place.
Figure 7-10 All Devices Receive the Frame, but Only the Destination Processes It
Any device that is the source or destination of an Ethernet frame will have an Ethernet NIC and, therefore, a MAC address. This includes workstations, servers, printers, mobile devices, and routers.
Unicast MAC Address (7.2.4)
In Ethernet, different MAC addresses are used for Layer 2 unicast, broadcast, and multicast communications.
A unicast MAC address is a unique address that is used when a frame is sent from a single transmitting device to a single destination device.
In Figure 7-11, the destination MAC address and the destination IP address are both unicast.
A host with IPv4 address 192.168.1.5 (source) requests a web page from the server at IPv4 unicast address 192.168.1.200. For a unicast packet to be sent and received, a destination IP address must be in the IP packet header. A corresponding destination MAC address must also be present in the Ethernet frame header. The IP address and MAC address combine to deliver data to one specific destination host.
The process that a source host uses to determine the destination MAC address associated with an IPv4 address is known as Address Resolution Protocol (ARP). The process that a source host uses to determine the destination MAC address associated with an IPv6 address is known as Neighbor Discovery (ND).
Broadcast MAC Address (7.2.5)
An Ethernet broadcast frame is received and processed by every device on an Ethernet LAN. The features of an Ethernet broadcast are as follows:
- It has the destination MAC address FF-FF-FF-FF-FF-FF in hexadecimal (or 48 1s in binary).
- It is flooded out all Ethernet switch ports except the incoming port.
- It is not forwarded by a router.
If the encapsulated data is an IPv4 broadcast packet, this means the packet contains a destination IPv4 address that has all 1s in the host portion. This numbering in the address means that all hosts on that local network (broadcast domain) receive and process the packet.
In Figure 7-12, the destination MAC address and destination IP address are both broadcast addresses.
Figure 7-12 Broadcast Frame Transmission
The source host sends an IPv4 broadcast packet to all devices on its network. The IPv4 destination address is a broadcast address, 192.168.1.255. When the IPv4 broadcast packet is encapsulated in the Ethernet frame, the destination MAC address is the broadcast MAC address FF-FF-FF-FF-FF-FF in hexadecimal (or 48 1s in binary).
DHCP for IPv4 is an example of a protocol that uses Ethernet and IPv4 broadcast addresses. However, not all Ethernet broadcasts carry IPv4 broadcast packets. For example, ARP requests do not use IPv4, but the ARP message is sent as an Ethernet broadcast.
Multicast MAC Address (7.2.6)
An Ethernet multicast frame is received and processed by a group of devices on the Ethernet LAN that belong to the same multicast group. The features of an Ethernet multicast frame are as follows:
- It has destination MAC address 01-00-5E when the encapsulated data is an IPv4 multicast packet and destination MAC address 33-33 when the encapsulated data is an IPv6 multicast packet.
- There are other reserved multicast destination MAC addresses for when the encapsulated data is not IP, such as Spanning Tree Protocol (STP) and Link Layer Discovery Protocol (LLDP).
- It is flooded out all Ethernet switch ports except the incoming port, unless the switch is configured for multicast snooping.
- It is not forwarded by a router unless the router is configured to route multicast packets.
If the encapsulated data is an IP multicast packet, the devices that belong to a multicast group are assigned a multicast group IP address. The range of IPv4 multicast addresses is 224.0.0.0 to 239.257.257.257. The range of IPv6 multicast addresses begins with ff00::/8. Because a multicast address represents a group of addresses (sometimes called a host group), it can only be used as the destination of a packet. The source is always a unicast address.
As with the unicast and broadcast addresses, a multicast IP address requires a corresponding multicast MAC address to deliver frames on a local network. The multicast MAC address is associated with, and uses addressing information from, the IPv4 or IPv6 multicast address.
48 Bit Hexadecimal Mac Address Labels
In Figure 7-13, the destination MAC address and destination IP address are both multicast addresses.
32 Bit Hexadecimal To Decimal
Routing protocols and other network protocols use multicast addressing. Applications such as video and imaging software may also use multicast addressing, although multicast applications are not as common.