Every Device Has a Name Tag: MAC Address

What does MAC stand for?

MAC stands for Media Access Control. It has nothing to do with any brand of "Mac" computer. Phones, computers, TVs, game consoles and even smart light bulbs of any brand have a MAC Address, as long as they go online.

Where does the name tag hang? The Network Interface

Strictly speaking, a MAC Address doesn't belong to the "whole device". It belongs to each Network Interface on the device. A Network Interface is the part a device uses to connect to a network, such as a Wi-Fi chip or an Ethernet cable socket. In older computers it was usually a card plugged into the motherboard, which is why it is also called a NIC (Network Interface Card).

So:

• A laptop with both Wi-Fi and an Ethernet socket has two MAC Addresses.
• A phone that has only Wi-Fi usually has just one MAC Address, for Wi-Fi.

Reading the name tag: 48 bits and Hexadecimal

All data in a computer ends up as a long string of 0s and 1s.

• Bit: the smallest unit of data in a computer, which can only be 0 or 1.
• Byte: 8 Bits make 1 Byte.
• MAC Address: 48 Bits in all, which is 6 Bytes.

Writing 48 zeros and ones directly would be far too long and easy to copy wrongly. So we use Hexadecimal instead. It uses 16 symbols, 0 to 9 plus A to F, to represent numbers (A = 10, B = 11 ... F = 15).

One hexadecimal digit stands for exactly 4 Bits, so two hexadecimal digits make 1 Byte. For example, 5E = 0101 1110.

So a MAC Address is 12 hexadecimal digits, in six groups of two. 48 Bits can make 281,474,976,710,656 different name tags (about 281 trillion).

Diagram: a MAC Address has 6 Bytes. The first 3 Bytes are the OUI (Organizationally Unique Identifier), and the last 3 Bytes are numbered by the manufacturer. Each Byte is shown as two hexadecimal digits.

The two halves of the tag: the OUI

The IEEE is an international organisation that sets technical standards such as Ethernet and Wi-Fi, and it gives different OUIs to different manufacturers. After each OUI there are 24 bits, which can make 16,777,216 numbers. When a large manufacturer uses them all up, it applies for a new OUI.

With this division of work, even if two companies number their devices separately, they will never produce the same name tag.

The example addresses in this lesson Every example in this lesson comes from a block of addresses set aside for teaching materials and documentation (00:00:5E:00:53:00 to 00:00:5E:00:53:FF), and does not belong to any real product. The OUI 00:00:5E was actually assigned by the IEEE to an Internet standards body, not to a real manufacturer. This lesson only borrows it for demonstration.

Three ways of writing, one name tag

Different systems show a MAC Address in slightly different ways, but the meaning is exactly the same. Upper-case or lower-case letters make no difference either.

Delivering letters inside a LAN: the recipient and sender of a Frame

When data is sent inside a LAN, it is put into a Frame. A Frame is the "delivery packaging" for data travelling within a LAN, with a delivery label stuck on the front that says:

When a device receives a Frame, it looks at the recipient first:

• It's my name tag → keep it and open it.
• It's not my name tag → ignore it and throw it away.

Both wired Ethernet and wireless Wi-Fi use MAC Addresses to write the recipient and the sender. The relationship between a Frame and a Packet is covered in Lesson 5.

Unicast and Broadcast

• Unicast: the recipient is an ordinary MAC Address, so the letter goes to just one device. Most Frames in everyday use are Unicast.
• Broadcast: the recipient is written as FF:FF:FF:FF:FF:FF, which means all 48 Bits are 1. This special Broadcast Address means "every device in the same LAN". It only ever appears in the recipient field, and is never the name tag of any device.

When do we need a Broadcast? Usually when a device wants to find someone but doesn't know their name tag. For example:

• When a phone has just joined Wi-Fi and doesn't have an IP Address yet, it broadcasts: "Can anyone give me an address?" This is DHCP (Dynamic Host Configuration Protocol), in Lesson 8.
• When a computer knows another device's IP Address but not its MAC Address, it broadcasts: "Who has this address?" This is ARP (Address Resolution Protocol), in Lesson 6.

A Broadcast stays inside its own LAN. A Router doesn't pass it on to other networks. Otherwise the broadcasts of the whole world would flood into your home and the network would jam.

Diagram: the top half shows Unicast, where only the device with the matching name tag keeps the Frame and the others ignore it. The bottom half shows Broadcast, where every device in the same LAN keeps it, but the Broadcast doesn't cross a Router into other networks.

The name tag is only useful inside a LAN

A MAC Address only handles "getting to the right device inside the same LAN". When data has to leave your home and pass through a Router to other networks, the Router removes the old delivery label and sticks on a new one, so both the sender and recipient MAC Addresses change.

That's why, when you open example.com, the faraway Server generally can't see your phone's MAC Address. The job of "finding the way across networks" belongs to the IP Address, in the next lesson.

Can the name tag be changed? Random MAC Addresses

The original name tag is written in the chip of the network interface, but the operating system can choose to use a different address instead.

Why do that? For privacy. If a phone used the same name tag everywhere, the Wi-Fi at a shopping centre or a station could record "where this phone has been". So many systems now use a different Random MAC Address for each Wi-Fi network:

Handy tip: if the second digit of the first pair of digits in a MAC Address is 2, 6, A or E, it is a "custom name tag" and is very likely a Random MAC Address. For example ?2:…, ?6:…, ?A:…, ?E:….