As the first chapter noted, physical addresses originally represented a single physical interface on a host or other network device. As computing power increased, developers built several virtual computers, or virtual machines (VMs), on top of a single physical computer.

These VMs needed their own physical addresses so they could send and receive network frames, so virtual interfaces were created. The idea of a virtual interface, once invented, was applied to many other problems; virtual interfaces are now ubiquitous in computer networks.

Note
VMs were originally developed to allow many different users to time-share on a single large-scale computer, such as a mainframe or minicomputer. Developers transferred the idea of VMs from these larger computers to smaller computers (microcomputers, which we call desktop computers today) to build sandboxes and emulators. To play an arcade game on a computer, you need an emulator, which is essentially a VM. If you want to test code to make certain it does not contain a virus, running it in a sandbox, another kind of VM, is a good idea.

The term mainframe originated in the telephone industry. Engineers constructed large frames to hold the massive wiring, crossbar switches, and Strowger switches, required to build a telephone exchange. The frame at the center of a region was called the main frame and housed in the central office. Smaller frames called building distribution frames (BDFs) might be placed in larger buildings as well. The first large-scale computers relied on massive wiring and hence were built using frames like those used in building telephone networks; hence, the term mainframe bled over from the telephone to the computing world.

There are many kinds of physical hardware addresses, but the most common is the Institute of Electrical and Electronics Engineers (IEEE) EUI-48 format, illustrated in Figure 2-4.

Figure 2-4 The EUI-48 Address Format

Note
You might see the term MAC-48 address from time to time. MAC-48 is an older name for EUI-48; the IEEE has declared MAC-48 obsolete.

The EUI-48 address is 48 bits or 6 octets. Each octet is encoded as a pair of hexadecimal digits and often (though not always) displayed in sections divided by dashes. The guide is at https://www.acedexam.com/300-415-ensdwi-implementing-cisco-sd-wan-solutions/

Note
Octet and byte are often used interchangeably in information technology, but they are not always the same thing. A byte is the number of bits a given processor can hold in internal registers or can process at one time. In an 8-bit processor, a byte is 8 bits; in a 32-bit processor, a byte is 32 bits. An octet, on the other hand, is always exactly 8 bits. Byte, however, is often used to mean exactly 8 bits, regardless of the processor. Because these terms have overlapping meaning, you might need to verify which meaning is intended. Byte almost always means a set of 8 bits in networking documentation and standards.

A physical shipping address has multiple parts: recipient, house number, street name, city, region, and state. As noted in the first chapter, part describes a different geographic region.
The EUI-48 address format is broken up in the same way, but rather than describing different geographic regions, each part describes something about the address.
The eighth bit of the first octet is called the I/G bit. The I/G bit tells you what the scope of this address is. If the I/G bit is set to 0, this is a unicast address—an address of a single physical interface. If the I/G bit is set to 1, this is the address of a group of physical interfaces, or a multicast group.
Interfaces are never assigned an EUI-48 multicast address.

Interfaces are programmed to listen to these addresses by software; any individual host might or might not be listening to a particular multicast address.

The seventh bit of the first octet is called the U/L bit. The U/L bit tells you if the address is globally or locally unique. Globally unique means just what it sounds like: no other device in existence, even in space, should have this same address. Locally unique addresses were often assigned by network administrators way back in the mists of time.
The first half, or three octets, of the address, is the organizationally unique identifier (OUI). While the OUI is divided into a few different registries, the main thing you need to know is the OUI tells you who—the organization—assigned the address. If the U/L bit is set to 0, this address was assigned by the device’s manufacturer.
Globally unique numbers are globally unique because each manufacturer is given a block of addresses. Manufacturers assign a number from their pool of addresses to each device they build. So long as these manufacturers assign each number in their pool to precisely one device, every device made will have a unique address.
Note
Could we run out of EUI-48 addresses? In theory, yes, but it does not seem likely any time soon. Even with the two reserved—U/L and I/G—bits removed from the calculation, the EUI-48 address space has some 70 trillion possible addresses. If we do reach the end of the EUI-48 address space, it is possible to recycle older addresses, because devices generally have some expected lifetime. Most devices will be thrown away within 10 or 15 years of being manufactured.
Because the I/G and U/L bits are placed at the end of the first octet, you can always tell what kind of EUI-48 address you are working with by looking at the last digit of the first octet:
•If the first octet ends in a 0, 4, 8, or C, this is a globally unique unicast address.
•If the first octet ends in 1, 5, 9, or D, this is a globally unique multicast address.

There is a longer version of the EUI-48 address called, naturally enough, EUI-64. The EUI-64 address has the same format as an EUI-48 address, only two octets longer—or 64 bits.