MAC Addresses Explained with Examples

In a network, an address provides a unique identity to an end device. Unless an end device has a unique address, it can’t communicate with other devices in the network. A unique address enables an end device to send and receive data in the network. In the LAN network, a unique address is the combination of two addresses: the software address and the hardware address.

A networking reference model defines the standards, characteristics, definitions, and functionalities of the network. There are two popular networking models: the OSI Seven Layers model and the TCP/IP model. In both models, the software address and hardware address are defined at the network and data link layers, respectively. In both models, the network and data link layers are ranked third and second, respectively. Because of this, both layers are also known as layer 3 and layer 2, respectively.

Software address

The software address, also known as the network layer address or layer 3 address, identifies a network interface. This address is manageable and configurable. Based on network requirements and layout, this address can be configured and assigned to an end device. Almost all modern LAN implementations use the IP protocol in the network layer. The IP protocol uses the term IP address to define the software address.

Hardware address

The hardware address is also known as the data link layer address, or layer 2 address, or MAC (Media Access Control) address. The term MAC address commonly refers to the hardware address. Unlike the IP address or software address, this address can’t be configured or managed. When you purchase a new NIC (Network Interface Card) or any device with an onboard NIC, it comes with a preconfigured MAC address.

A MAC address is a 6-byte (48-bit) binary address. MAC addresses are written in the hexadecimal format. The hexadecimal format uses base 16 to refer to numbers. If you divide the total available length (48 bits) in binary numbers by the base (base-16) that is used to write a number in hexadecimal format, you get the total digits (12 = 48 ÷ 16) of that number in the hexadecimal format. Thus, if you write a 6-byte (48 bits) binary MAC address in hexadecimal format, you get a 12-digit hexadecimal number.

For convenience and easier readability, when writing a MAC address in hexadecimal format, extra spaces, periods, or colons are added after every two or four digits. For example, you can write a MAC address in the following ways.

• Without any separator: - 00000ABB28FC
• Extra space after every two digits: - 00 00 0A BB 28 FC
• Extra space after every four digits: - 0000 0ABB 28FC
• Colon after every two digits: - 00:00:0A:BB:28:FC
• Colon after every four digits: - 0000:0ABB:28FC
• Period after every two digits: - 00.00.0A.BB.28.FC
• Period after every four digits: - 0000.0ABB.28FC

No matter which style you use to write the MAC address, or an application or networking software uses to display the MAC address, a MAC address is always processed in binary numbers only. NIC converts the MAC address's hexadecimal values to binary before processing and using them.

Network requires unique MAC addresses

If a LAN network has two or more NICs configured with the same MAC address, then that network will not work. Here is an example to illustrate this.

Suppose in a network, three PCs: PC-A (11000ABB28FC), PC-B (00000ABB28FC), and PC-C (00000ABB28FC) are connected through a switch. NICs of PC-B and PC-C have the same MAC address 00000ABB28FC.

If PC-A sends a frame to the destination MAC address 00000ABB28FC, the switch fails to deliver it, as it has two recipients.

The following image shows this example.

A LAN network cannot function unless each device has a unique MAC address.

Structure or format of the MAC address

As mentioned above, you can’t assign a MAC address to a NIC or onboard NICs. When you purchase a new NIC or a device with an onboard NIC, it arrives with a preconfigured MAC address or MAC addresses, respectively.

Before manufacturing NICs, every manufacturer obtains a universally unique 3-byte code, known as the organizationally unique identifier (OUI), from the IEEE. The IEEE is an international organization that governs and maintains the MAC address namespace.

After obtaining the OUI bytes, the manufacturer uses them at the beginning of the MAC address for all its NICs or on-board NIC devices. The manufacturer also assigns a unique hexadecimal value in the remaining bytes.

6 bytes MAC address = 3 bytes OUI number obtained from the IEEE + 3 bytes unique number assigned by the manufacturer

MAC addresses of all NICs, including onboard NICs, manufactured by the same manufacturer always start with the same 3-byte OUI. For example, suppose the IEEE assigns an OUI “0000AA” to the xyz company. Now, the XYZ company will use the OUI number 0000AA as the first 24 bits to build MAC addresses for its NICs or onboard NICs.

To keep each product separate from others, the manufacturer uses the remaining 3 bytes. Manufacturers are free to use any sequence or method on the remaining three bytes. For example, the XYZ company can assign the MAC addresses to its NICs in incremental order.

The following table extends this example and adds two more demo companies (ABC and JKL). It also shows the MAC addresses of 5 NICs from each company.

Thus, this procedure ensures that no two NICs use the same MAC address in the universe.

Types of MAC addresses

There are three types of MAC addresses: unicast, multicast, and broadcast.

Unicast MAC address

A unicast MAC address represents a specific NIC or onboard NIC ports in the network. A NIC's built-in MAC address is its unicast MAC address.

Multicast MAC address

A multicast MAC address identifies a group of devices (or Layer 2 NICs). The IEEE has reserved the OUI 01-00-5E (the first 3 bytes, or 24 bits) for multicast MAC addresses. The remaining 24 bits are set by the network application or device that wants to send data in the group. A multicast MAC address always starts with the prefix 01-00-5E.

Broadcast MAC address

A broadcast MAC address represents all devices on the network. The IEEE has reserved the address FFFF.FFFF.FFFF as the broadcast MAC address. Any device that wants to send data to all devices of the network can use this address as the destination MAC address.

Conclusion

MAC addresses provide a unique identifier for each device on a LAN, ensuring reliable communication and preventing address conflicts. Understanding the structure, types, and assignment process of MAC addresses helps you better manage and troubleshoot your network devices.