Lesson
Logical and Physical Addressing
Learning objective
Distinguish MAC and IP address roles and recognize their basic formats.
Why networks use addresses
You already know that a laptop, printer, or server joins a network through a network interface. An interface can have more than one kind of address. A Wi-Fi interface and an Ethernet interface on the same laptop are separate interfaces.
Two important forms are MAC addresses, used as local-link identifiers, and IP addresses, used as logical network addresses. They are related but not interchangeable.
| Address | Beginner meaning | Example |
|---|---|---|
| MAC — physical addressing | An interface's identity on an Ethernet or Wi-Fi link | 02:1A:2B:3C:4D:5E |
| IP — logical addressing | An address assigned to an interface for a particular IP network | 192.0.2.10 |
One laptop can use both at the same time. Physical does not mean a label on the cable, and logical does not mean imaginary: both are real values used by networking software.
MAC addresses
MAC stands for Media Access Control. An Ethernet MAC address is 48 bits and is commonly written as six hexadecimal pairs, such as 02:1A:2B:3C:4D:5E. Hexadecimal uses digits 0–9 and letters A–F.
Each pair represents eight bits, also called an octet. Six octets make 48 bits. You do not need to convert a whole address to binary to recognize the format.
A MAC address identifies an interface for local-link purposes. A laptop's Wi-Fi and Ethernet interfaces normally have different MAC addresses. It is more precise to say “this interface's MAC address” than “this computer's one permanent address.”
A manufacturer can assign a MAC address, but software can also assign or change it. Many devices use a randomized MAC address for privacy. Calling MAC addressing physical addressing does not mean that every MAC address is fixed forever.
U/L and I/G bits
Two bits in the first octet describe the kind of MAC address. In the usual written binary form, look at the two bits on the right:
| Bit | Position | If it is 0 | If it is 1 |
|---|---|---|---|
| I/G — Individual/Group | Rightmost bit, bit 0 | Individual address, also called unicast | Group address, used for a group of receivers |
| U/L — Universal/Local | Next bit to the left, bit 1 | Universally administered | Locally administered |
The I/G bit tells you whether the address is for an individual interface or a group. A group address is not an ordinary host interface address. Later lessons explain delivery to groups.
The U/L bit describes how the address is administered. Universal addresses use an IEEE-assigned prefix; local addresses can be assigned through software or by a network administrator. This is also sometimes called the Local/Global bit. “Local” here does not mean “my house,” and “universal” does not mean “reachable over the Internet.”
For 02:1A:2B:3C:4D:5E, the first octet 02 is 00000010 in binary. The I/G bit is 0 and the U/L bit is 1, so it is an individual, locally administered MAC address. The explorer below lets you change just these two bits.
IP addresses
IP stands for Internet Protocol. An IP address is a logical network address assigned to an interface. A host can have more than one IP address, and its IP addresses can change when it joins a different network.
There are two versions to recognize:
- IPv4: 32 bits, commonly written in dotted decimal, such as
192.0.2.10. There are four numbers separated by dots, and each number is between 0 and 255. - IPv6: 128 bits, written as hexadecimal groups separated by colons, such as
2001:db8::10. The double colon is a shorthand for consecutive zero groups.
Imagine using the same laptop interface on two different networks. It can keep the same MAC address while receiving a different IP address. That distinction is the main idea to remember; real devices may also change their MAC addresses.
MAC addresses support local-link identity; IP addresses provide logical network addressing across connected networks. Detailed IPv4, IPv6, and network-prefix concepts come in the Network Layer module. The addresses in this lesson are teaching examples.
Inspect address formats
First compare the same interface on two example networks. Then choose an address sample to recognize its format. For MAC examples, toggle the U/L and I/G bits and observe the first octet and its meaning change.
Explore MAC and IP addresses
MAC and IP addresses have different jobs and recognizable written formats.
- Physical address · MAC
02:1A:2B:3C:4D:5E- Logical address · IPv4
192.0.2.10
Switch networks to see the IP address change. In this example, the same interface keeps its MAC address. Real devices can also change or randomize their MAC addresses. These are teaching addresses.
Inspect an address format
Choose a sample. For a MAC address, try changing the two highlighted bits.
Knowledge check
The bit terminology is based on RFC 9542, Special First Octet Bits.